The Hu m an Eye and Vision-I: Producing the Image

CHAPTER 5

5.1

-

INTRODUCT ION

Our eyes have evolved to deal wi th a remarkable variety of situations and tasks: from gUiding us down a dark path on a moonless night, to helpin g us choose in the noonday sun w hich subtly colored mush­ room is safe to eat, to gauging the distance to a tree branch as we swing through the air. The variety of our visual experience is im­ mense: brightness, color, form , tex­ ture, depth, transparency, motion, size. and so on. The deep link be­ tween vision and knowledge is cap­ tured in our language: "See what we mean?" The importance of the eye is re­ flected throughout our culture. For thousands of years, the human eye has been revered as an organ of mystery, imbued with power, not only to help, us perceive the world, but also to charm or influence peo­ ple and events. The pervasive leg­ end of the evil eye reflects this fas­ Cination. The legend probably arose from the belief tha t there are rays emitted by the eye that can influ­ ence others as well as "touch" the world. In the first millennium B.C . in Mesopotamia, the legend in­ volved certain demons, called utukku , who haunted and spooked deserts and graveyards and whose power lay in their glance, which could Injure anyone unlucky enough to get too close. Saint Mark groupe d the evil eye among the low­ est of human traits: "adulteries,

fornications, murders, thefts , cov­ etousness, Wickedness, deceit, las­ cIVlousness, an evil eye, blas­ phemy." Marcus Terentius Varro (116-27 B . C.) claimed that the evil eye in women arose from unbridled passions . The Hindu god Shiva could consume objects in flames with a look from his third eye. A glance from an evil eye could ruin crops, cleave rocks, split precious stones, transmit disease (as Shake­ speare wrote, "They have the plague and caught it of your eyes"), ad­ versely affect sexual function, and rob Christians of faith. Death, too, is a common result of a glance from an evil eye. The beautiful slave girl Twaddud in The Arabian Nights at­ tracted men and then "shot them down with the shafts she launched from her eyes." Two Moroccan prov­ erbs quantify this: "The evil eye owns two-thirds of the graveyard " and "one-half of mankind die from the evil eye." Perhaps evil eyes were particularly virulent in third-cen­ tury Babylon, where the TalmudiC scholar Rab attributed 99 out of 100 deaths to the evil eye. Happily, modem adaptations of this my­ . thology depict a powerful benefactor using his vision to further the cause. of 'Truth, Justice and the American Way" (Fig. 1.1). The eyeball itself was thought to posess magical powers. Jerome Cavdan, a professor of medicine in the mid-sixteenth century, dis­ cussed the belief that by holding the eyeball of a black dog in your hand, you could prevent the neigh­

144

borhood dogs from barking. Natu­ rally, such eyeballs were prized by thieves. As late as the mid- 1930s, some of the Pennsylvania Dutch b e­ lieved that pinning the eye of a wolf to the inside of a sleeve could pro­ tect the wearer from accidents. (The method of pinning is unclear.) To­ day, psychiatrists treat ophthalmo­ phobia-fear of being stared at. But eyes need not be evil. For ex­ ample, Cervantes calls them "those silent tongues of love. " "Love had his dwelling in the subtle streams of her eyes ," writes Chaucer in "Troilus and Criseyde." The Polish poet Daniel Naborowski expresses this more romantic view: "The Sin­ gle word 'eye' embraces all:rrhe torches, stars, suns, firmaments, and gods. " Here we shall be concerned not with the eye's power for good or evil, but rather with its power fo. Vision. What does it do, and how does it do it? This concern with the physical mechanism of the eye led Rene Des ­ cartes, in the seventeenth century, to demonstrate and explain the im­ age-forming properties of the eye, and it is this approach that well pursue. We'll begin by relating th e eye to something familiar: the cam­ era (Chapter 4). Though there ar e important Similarities betwee n the eye and camera, they do not tell the whole story. In this chapter we 'll stress these similarities , then re­ turn to the eye in Chapter 7 to con­ Sider properties of the eye (and brain) that are rather different from those of the camera.

5.2 EYE AND CAMERA

145

5.2

-

Retina

EYE AND CAMERA

T here are a number of similarities betwee n a human eyeball and a simple camera (Fig. 5.1 J. We've seen that a camera is a light-tight box with a lens system for forming a real image on the light-sensitive film. Stops and a shutter are used to control the amount and duration of the light admitted . The eye, too, is basically a light-tight box, whose outer walls are formed by the hard, whi te sclera. * There is a two-elemen t lens system, consisting of the outer cornea and the inn er crystalline lens or eyelens. t The lens system forms an inverted image on the ret­ ina ~ at the back of the eyeball. The colored iris corresponds to the dia­ phragm in a camera. Its pupil.§ the dark, circular hole through which the I1ght enters, corresponds to the camera aperture. Though not anal­ ogous to a shutter, the eyelid is like a lens cover, protecting the cornea fro m di rt and objects. The eye or tear flu id cleans and moistens the cornea and can be compared to a lens cloth or brush. The volume of the eye is not empty, as is that of the camera, but is filled With two transparent jelly­ like liq uids, the vitreous humor and aqueous humorll which pro­ vide n ourishment to the eyelens and cornea. Additionally, their in­ ternal p ressure helps to hold the s hape of the eyeball. (Glaucoma is a

Eyelens

Iris

Aqueous humor

(a)

Temporal si de

Iris

1= "..",..,...-.5

Optic nerve

Nasal side

(b)

'Greek, skleros. hard. At the Battle of Bunker Hill, Colonel William Prescott is said to have shouted, "Don't shoot until you see their scleras."

Film

tThis convenient terminology comes from Frans Gerritsen's The Art oj Color. fLatin rete. net-the nerve cells of the retina give the retina an appearance of a net. §Latin pupa. doll. You see a tiny image of yourself in another's eye (you doll!). IILatin vitreus, glassy, and aqueus, water, plus umor. flUid. Actually, the humors are somewhat gelatinous. neither as dense as glass. nor as fluid as water.

FIGURE 5.1 Human eyeball: (a) in three dimensions, (b) in two-dimensional cross section, and, for comparison, (c) a simple camera .

(e)

CHAPTER 5: THE HUMAN EYE AND VISION-l: PRODUCING THE IMAGE

14 6 disease in which the pressure of these fluids is too h igh.) To see evi­ dence of the fluid nature of these humors, stare at a large. bright, uniform area, such as a blank sheet of well-lit white paper or the sky on a cloudless day. Move your eyeball from side to side quickly. then hold It fixed. The moving dots and chains you see are shadows due to dead b lood cells floating in your hu­ mors. The older you get. the more of these floaters you will have. P ON DER

Why are the humors, rather than blood vessels, used to nourish the cornea and eyelens?

The lack of blood vessels in the cor­ nea and eyelens means that, when surgically transplanted, these ele­ ments are less likely to be rejected by the immune system. which re­ sides in the blood. Nowadays. cor­ nea transplants are performed rou­ tinely. The "film" of the eye. the retina. contains over a hundred million light-sensitive cells. the rods and cones, packed together like the chambers of a honeycomb. Near the center of the retina-in a small re­ gion called thefo l1ea*-there are a great number of cones and no rods. This region is responsible for the most precise vision. and also for color vision (Plate 5.1 and Fig. 5.2). Away from the fovea rods predomi­ nate. and at the extremes of the ret­ ina, corresponding to peripheral vi­ sion. there are very few cones. The rods and cones are connected through a network of nerve cells in the retina to the op tic n erve. There are only about one million nerve fi­ bers in the eye's optic nerve; thus each such fiber is connected to m any rods and cones. There are no rods or cones at the point of the retina where the optic nerve leaves the eye, so you cannot see light that strikes this spot-the blind spot. You can find your blind spot by using Figure 5.3.

'Latin, depression. small pit. The fovea Is a shallow depression in the retina.

PONDER

Is the blind spot on the part of the retina near the nose or near the ear?

Upon learning of the blind spot, Charles If is said to have amused himself by looking to the side and making the heads of his courtiers disappear. As you might expect. the exis­ tence of a special region of sharp. detailed viewing near the center of the retina results in some Signifi­ cant differences in behavior of the eye as compared to the camera. A camera should be held fixed to pro­ duce an unblurred image. and then

each part of the exposed fil m is equally sharp and detailed. Bu t your eye must scan-move to po int in different directions-so tha t dif­ ferent parts of the scene fall on your fovea, the small region of your sharpest Vision. In this way. you can build up a sharp. detailed, mental picture of the world withou t having large numbers of optic ne rve fibers attached to all parts of t h e retina. The effiCiency of scanning is illustrated by the eye of the tiny ma­ rine organism CopUia (Fig. 5.4). Each of its eyes has only one light­ sensitive cell, which scans across the focal plane of the lens in front of it every second or so. Rather than getting a complete picture of the world all at once. the CopUia brain (such as it is) receives a sequence of messages telling how the int-": lsity of the light changes as the cell senses different directions . With this information. CopUia builds up

FIGURE 5.2

In this fluorescein angiograph, the blood vessels in the retina are made visible by injecting a fluorescent substance into the blood system. Note the fine network of blood vessels, except over the fovea.

FIGURE 5.3

Locale your blind spot. At a distance of about 25 cm, close your left eye and stare at the X with your right eye. Slowly move the tip of your pen across the page toward the right .from the X. Keep looking directly at the X, viewing the pen with your peripheral vision. At about 8 cm to the right of the X, the tip of the pen will no longer be visible: when its image falls on the blind spot. You can "map out" the extent of thetblind spot by placing dots on the page where the tip of your pen becomes invisible.

x

fiGURE 5.4

Photograph of the tiny (millimeter size) sea animal Copilia. Each eye contains an outer (corneal) lens and an inner lens attached to the photoreceptor. The inner lens and its associated receptor sweep side to side while sending signals through a single optic nerve to the brain.

5.2 EYE AND CAMERA

147

its mental picture of the world. The wiring efficiency of the scanning process is the reason that TV cam­ eras also scan. The scanning there is faster and more orderly-and done electronically, rather than op­ tically-to produce a sequence of electronic signals that can be trans­ mitted to your home TV or stored on tape. In other animals, the nature of the region of sharp, detailed view­ ing in the retina depends on the needs and habits of the animal. Birds, for example, have a special p roblem; their eyes are set into the sid es of their heads, giving them a maximum field of view. However, in addition to side vision , a bird must have sharp forward vision to see where it is flying. Many birds solve this problem by having two "central areas" (similar to our fovea) in each eye-one close to the optic axis for sharp side vision (recall how a pi­ geon looks at you), and one toward the back to provide sharp forward vision. Rabbits' eyes are different; they have a foveal "strip," which gives them excellent acuity where danger or predators are most likely to be-on the horizon.

the focusing. The shape of their cornea is not important and differs in different fish. If an animal is to see both in air and in water, the cornea should be flat to provide no fOCUSing for distant objects in ei­ ther case. Crustaceans and some ducks have such eyes. The anableps , a fish with a face that only its mother could love, solves the problem of simultaneous vision in air and water by a differ­ ent technique (Fig. 5 .5J-a bifocal eye!

(a)

A. Foc using and accommodation The cornea and eyelens form a lens system similar to that of Figure 3.31, and produce a real, i.nverted image on the retina. (The first TRY IT tells you how to verify that it is inverted. ) The amount of bending of light at each surface depends on the ratio of the indexes of refraction of the two media involved, in accor­ dance with Snell's law. For this rea­ son, most fOCUSing occurs at the air-cornea surface (naIr = 1'.000, ncornea = 1.376). while less occurs at the surface of the eyelens, whose in­ dex of refraction is only sligh tly greater than that of the surround­ ing humors (neye lens ranges from 1.386 near its surface to 1.406 near its center, whereas nhumo rs 1.336). Because ncornea is very close to nwater> there is almost no bending of light at the cornea when the eye is underwater. Thus, fish have al­

most spherical eyelenses that do all

(b)

FIGURE 5.5 (a) Photograph of an anableps. (b) The anableps eye at the water's surface, looking upward and downward simultaneously. Rays from the airborne prey are focused by both the air-cornea surface and the gently curved (long focal length) part of the eyelens. Rays from the underwater predator are not focused at the water-cornea surface but are focused by the sharply curved (short focal length) part of the eyelens.

CHAPTER 5 : THE HUMAN EYE AND VISION-I : PRODUCING THE IMAGE

148

P O N DER How do goggles help you see undelWater? (Goggles are made of a flat transparent pie ce of plastic held in front o f your eyes with a watertig ht seal.)

Only low-quality cameras have fixed lens systems; better cameras can be adjusted to focus on distant or on nearby objects. In the camera, adjusting the focus is achieved by changing the lens-film separation­ increasing it for closer subjects (Sec. 4.2Bl. Our eyes do not change focus this way, but the eyes of many fish do. Changing focus in a camera is necessary when the depth of field is limited . You can easily check that your eyes also have limited depth of field , a nd that you can change your focus for near and far objects. Hold up your thumb in front of a distant object such as a bookcase across your room. Focus on your thumb with one eye, clOSing the other. Without refocusing or pointing your eye in a different direction, notice that the bookcase is out of focus . Next, keep your thumb where it Is, and focus on the bookcase; now the thumb is out of focus. This demon­ s trates that the eye has only a fair depth of field, and that you can change your focus for objects at dif­ ferent distances. (Making the aper­ ture smaller, of course, increases the depth of field . Repeat the exper­ im ent while looking through a pin­ hole In a piece of aluminum foil held close to your eye. The improved focus and depth of field may be suf­ ficient to give you a sharp image even if you remove your glasses.) You often make use of your limited depth of field when driving a car in the rain. You lean close to the wind­ s hield to make the spattered drops close to you , and thus out of focus while you are properly focused on the d istant signs. Likewise, specks of d irt on your eyeglasses are not too n oticeable . Your ability to change focus de­ pends on the fact that your eyelens is rather elastic . Instead of chang­ ing the lens-retina separation as the fis h does, you change focus by

changing the Jocal length oj your eyelens-a process called accom­ modation. This is performed by the ciliary muscles. When these mus­ cles are relaxed, the eyelens is pulled out by the suspensory liga­ ments and has the shape shown in Figure 5.6a-a long focal length for viewing distant objects. When the ciliary muscles are tense they form a smaller ring, thus releasing the tension in the ligaments and allow­ ing the elastic eyelens to bulge into the shape that it prefers (Fig. 5.6bl-with a shorter focal length for nearby objects. (To see these changes in lens shape , refer to the second TRY IT. 1 Greater precision is possible by making a large adjust­ ment In a weak element (the eye­ lens). rather than a small adjust­ ment in a powerful element (the corneal-just as you can set your UHF TV station more accurately by adjusting the fine tuning knob carefully. The strain you feel in your eyes after hours of dOing close-up work is the fatigue of the ciliary muscles (and alSo , perhaps, the fa-

FIGURE 5.6

Accommodation . (a) Relaxed ciliary , muscles allow the suspensory ligaments to stretch th e eyelens , which then has a long focal length for viewing distant objects. (b) Tens e ciliary muscles release the suspensory Iigaments and the eyelens bulges for vi e wing near objects.

tigue of muscles outside the eye tending to "cross" your eyes for close work-Sec. 8 .3). Nonnal eyes are able to accom­ modate for object distances between infinity and about 25 cm, but if the cornea bulges too much or too little, the eyelens may not be able to bring the image to a focus on the retina. For Instance, a sharply b ulg­ Ing, short-focal-Iength cornea m ay be proper for focusing on nearby objects , but too strong fo r distant objects , no matter how r elaxed th e ciliary muscles are-a condition called myopia. * Similarly, too long a focal length produces hyper­ opia. t A person with either of t h ese conditions may require eyeglasses or contact lenses to compensate (Sec. 6 .2l. (A more extreme meth od of correcting myopiC vision involves surgically scoring the cornea sur­ face. This process , radial kerato­ tomy, relaxes the internal stres s in the cornea, allowing it to assume a '

*Greek, mUD , shut or close, plus DPS, eye. People with myopia sqUint when trying to see distant objects without glasses . By reducing the aperture, they increase the depth of field as with the pinhole. t A contraction of "hypermetropia," from the Greek hupennetros, beyond measure plus ops. The hyperopic eye

can fo cus on very distant objects.

~~~====:]]t~~~

Di stant ob ject

(a)

(b)

5.2 EYE AND CAMERA

149 flatter shape. Other processes, suit­ able for both conditions, involve carving or molding the cornea into the desired shape.) Occasionally, particularly as one gets older, the eyelens may develop an opaque white cloudiness-cata­ racts. Light can no longer reach the retina. In this case, the eyelens Is surgically removed and the focusing power of it is replaced by glasses, a contact lens, or an artificial lens surgically implanted. While the eye then can see, it no longer has the ability to accommodate that it pre­ viously did.

First TRY IT fOR SECTION S.2A

The orientation of the retinal image You can easily tell that the image on your retina is inverted by touching your eye. Open one eye and look at a piece of white paper. Use your little finger to touch gently the outside of the lid near the corner of the open eye, by the nose . You will see a dark spot off to the side toward your ear. The gentle finger pressure on the eye restricts blood flow in the retina at that position, and the retinal cells cannot respond. Thu s, there is an apparent black spot. The spot you see is toward the side opposite the finger because the image on the retina is inverted; light from the side near your ear is imaged on the part of your retina near your nose . If the image is inverted, why do you see the world as " upright' ? In Chapter 7 we'll see that your sense of sight corresponds to complex patterns of activity in your brain-activity that is not simply related to the image on your retina itself. The relationship between your sensation of sight and this neural activity is, at best, problematic. It is meaningless to compare rightside up and upside down between material thing s, such as trees , and the immaterial, subiective sensation of your sight of the tree. What is important is that all your subiective se nsations agree as to the location of objects. You need only bump into a few th ings for your brain to achieve this agreement. If you wear goggles that invert the retinal image fro m the usual orientation, you can learn to ride a bicycle, iump rope , play catch and so on-evidence of the remarkable ability of your brain to correlate sight with physical experience.

Second TRY IT fOR SECTION S2A

Accommodation With a candle, a dimly lit room, and an accommodating friend, you can study the action of the eyelens. Hold the lit candle about! meter from your friend's eye, slightly to the side of her direction of gaze. Look carefully at the reflection s of the candle in her eye. You should see something like Figure 5.7. These are the Purkin;e images. The first, which is the brightest, is due to the ou ter corneal surface . The image is erect, virtual, and smaller than the object. The third Purkinje image, due to the front of the eyelen s, is also erect, though somewhat dimmer than the first. The fourth Purkinje im age is due to the rear surface of the eyelens. (The second Purkinje image, due to the inner surface of the cornea, is probably too faint to be seen.)

Have your fri end accommodate by focusing on something close, say your ear. Note carefully the positions of the Purkinje images. Now have her focus on a distant object, without changing her direction of gaze. Only the fourth Purkinje image shifts significantly, because accommodation is achieved by changing the curvature of the rear surface of the eyelens (see Fig. 5.6) .

PONDER

The first, third , and fourth Purkinje images. The third image is blurred because it is reflected from the pebbly front surface of the eyelens . The fourth (inverted) Purkinje image moves during accommodation, implying that the rear surface of the eyelens changes shape.

Why are the first and third Purkinje images erect (a nd virtual) while the fourth is inverted (a nd real)?

*B. Aberrations

As in any image-forming device, there are aberrations associated with the lens system of the eye. Na­ ture has been very clever in mini­ mizing these aberrations, and the few aberrations that are not well

FIGURE 5.7

corrected tum out to be of little consequence. The fovea is the only region where aberrations need to be reduced to a minimum, as it is the region of your most precise viewing. It lies nearly on the axis of the lens system of the eye, where the effects of all

CHAPTER 5, THE HUMAN EYE AND VISION-I, PRODUCING THE IMAGE

150 aberrations are smallest. Although pOints on the retina away from the fovea are more subject to the effects of aberration and the image is con­ stantly degraded there, that is not very critical-you can always check any fuzzy image there by moving the fovea Into the region of Interest.

Aberrations are also reduced in other ways. In the camera, curva­ ture offield (Sec. 3.5D) Is a problem as the image plane should lie on the flat film. In the eye, the retina is al­ ready curved , reducing this prob­ lem. The eye avoids spherical aber­ ration (Sec. 3.5B) in several ways. First, the cornea, where most of the focusing is done, is not a spherical lens-it Is flatter, less curved near its periphery than a spherical lens is (Fig. 5.8) , so there is less bending of the peripheral rays, as desired. Furthermore, the eyelens has nei­ ther a spherical shape nor a uni­ form index of refraction. The eye­ lens is made in layers, like an on­ ion , with the inner layers formed first and the outer layers added as the eye ages (Fig. 5.9). This makes the index of refraction lower at the periphery, so there is less focusing of the peripheral rays-less spheri­ cal aberration. We saw (Sec. 3.5D) that distortion is reduced when the stop is between the elements of a two -lens system; in the eye, the iris (stop) lies between the cornea and eyelens.

FIGURE 5.9

The eye does not have an achro­ matic doublet to reduce chromatic aberration (Sec. 3.5A). Although the material of the lens system doesn't have much dispersion be­ tween the red and the blue-green regions of the spectrum, chromatic aberration is appreciable in the blue and ultraviolet (lNl. so the eye tends to discard the latter wave­ lengths. The eyelens absorbs the IN as well as some blue. (With age, the eyelens becomes yellower-it ab­ sorbs more blue light. It has been suggested that most artists use less blue during their later years be­ cause they see less blue. However, if the eyelens is removed, say for cat­ aracts, even ultraviolet light be­ comes visible.) Additionally, very lit­ tle blue light is processed by the fovea where fine focus Is most im­ portant. The fovea is populated by cones, which, taken as a group, are most sensitive to the yellow-green and somewhat less sensitive to the blue. On top of that (literally) lies the macula lutea, * a small, yellow­ pigmented layer that covers the fovea, further rejecting blue light. The corrections for chromatic ab­ erration are not perfect. though, as you can easily see-"black lights" look fuzzy as a result of chromatic aberration. The eye focuses on the little amount of yellow-green lighl present in these IN lights, wh tle the enormous amount of blue gets through the lens and macula lutea and is detected by the ·cones. Be­ cause it is out of focus, you see a blue fuzz around the bulb. Alterna­ tively, you may notice a thin light line at the border between the bright red and blue regions of Pla te 5 .2. This line is not present in the painting or the reproduction and is due to chromatic aberration. Since your eye cannot focus on the red and blue simultaneously, at least one region is slightly blurred. Thus, there is a line on your retina receiv­ Ing light from both red and blue re­ gions; the result is a light line. An­ other example uses the meters on certain stereo eqUipment that have

Eyelens consisting of layers of transparent fibers in clear elastic membrane. The lens of an 80 year old is H times as thick as that of a 20 year old.

'Latin macula, spot (cf immaculate = spotless). plus luteus, yellowiSh.

Fibers

(b) membrane

FI GURE 5 .8

(a) The cornea is less curved at its periphery and thereby reduces spherical aberration. The index of refraction of the eyelens is lower near its surface than at its center, also helping to reduce spherical aberration. (b) Photograph of the cornea from the side.

5.3 THE RETINA

151

some figures glowing In red and others in blue. Focus on lit figures of one color in a dark room and youl1 notice that the other figures are out of focus-they may pulsate, alternately In and out of focus. Probably the most effective com­ pensation for aberrations is the complex learning and processing done by the brain. For Instance, if you wear goggles that have a great deal of chromatic aberration, you soon learn that the colored edges around Objects are not real. Your brain compensates, and after a few days , you don't experience any col­ ored edges. When the goggles are removed, however, you see colored edges even though they are not physically present in your retina. Another experiment uses glasses in which each lens is blue on the left half and yellow on the right half. with a vertical line dividing the two areas. After a while, you learn to ig­ nore the blue and yellow half-worlds and sense no difference from your usual perception-you perceive no color or boundary. Similarly, mil­ lions of wearers of bifocal glasses learn to ignore the line that divides their field of view. These examples of effects more drastic than optical aberrations demonstrate the re­ markable ability of your brain to compensate for unusual images on the retina.

C. The iris Like the diaphragm in a camera, your iriS opens and closes in re­ sponse to the average ambient light level. (You can easily check this with the TRY IT.) When your iris is fully open, your eye has an f-num­ ber between fl2 and [/3 (compared with flO.9 for the nocturnal cat, and f/4 for the diurnal pigeon). Your iris controls light admitted to your eye fairly rapidly, in about t second, but the amount of light reduction is rather small. less than a factor of 20. The main function, therefore, cannot be to control the light inten­ sity, since the range of light inten­ sities to which you can reasonably respond is enormous (about a fac­ tor of 10 13 ). Instead, in bright con­

ditions your iris stops down to re­ duce aberrations and increase the depth of field. When you do close work, such as threading a needle, your iris tends to stop down. This increases your depth of field and hence allows you to focus on objects closer to you than you otherwise could focus on. It also diminishes the need to keep changing accom­ modation. As you grow older and your viSion deteriorates, your Iris will stop down under a greater va­ riety of conditions to increase depth of field and reduce aberrations. Un­ der low light levels, when your eye can't afford these luxuries, the iris opens up allOWing more light in. This occurs at times when gather­ ing light is more important than a sharp image. For example, in the dark it is more important that you be able to see that there is a tiger present, than being able. to tell its sex. We have the same trade-off­ between light gathering power and resolution (the abJlity to see fine detail)-that we saw in camera film (Sec. 4.7G) and that well see again and again.

TRY IT FOR SECTION S.2e

The iris To see some properties of th e action of the iris, all you need is a flashlight and a friend (or a mirror). In a dim room, shine the flashlight into your friend 's right eye and notice the contraction of his iris. Turn off the light and watch his iris open again. Without touching his eye, use a plastic ruler marked in millimeters to measure the diameter of his pupil in each case. Using the fact that the area of a disk is proportional to the square of the diameter, calculate the ratio of the pupil areas in the two cases. Now shine the light in his left eye while watching his right eye; his right iris responds. The brain causes the irises to operate together. Hold the flashlight about ~ m directly in front of him, and notice the size of your friend 's pupil. Now turn on an additional light, dimmer (or more distant) than the first, but somewhat to the side, and observe Fechner's paradox. Although there is more light entering your friend's eye, his pupil opens up

further! The size of the pupil is determined by the average light over the illuminated area of the retina. Even though there is more total light present in the latter, two-light case, it is spread out over more of the retina, so the average is lower (and the iris opens) . The size of the iris also depends on the state of accommodation. Under normal room illumination , have your friend first focus on something near and then on something far. Notice that the iris stops down when he focuses on the close object, even though there is the same average illumination. The smaller pupil increases the depth of field and allows him to see quite close objects in good focus .

5.3

-

T HE RETINA

The retina is, in many ways, analo­ gous to the film in a camera. Each receives a real image, the first step in a complicated process that re­ sults in either a photograph or our sense of Sight. Figure 5.10 shows a schematic cross section of a human retina, with its three layers. The light sensitive layer consists of a dense array of rods and cones (the photoreceptors), which absorb the light. The plexiform layer* con­ sists of several types of nerve cells 'From the Latin plectere, to braid, to plait. A plexus has come to mean a network.

''I"'

nerve

FIG URE 5.10

Layers of the human retina (at a point other than the fovea).

CHAPTER 5: THE HUMAN EYE AND VISION-I: PRODUCING THE IMAGE

152

that process the signals generated by the rods and cones and relay them to the optic nerve. Finally. the choroid carries major blood vessels to nourish the retina and also acts as an antihalation backing. absorb­ ing light so that it cannot reflect and strike the rods and cones on a second pass. Our pupils look so dark because much of the light that enters them is absorbed by either the photore­ ceptors or the choroid. If a bright light is shined directly into the eye , though, some will come back out. This is why the eyes of subjects in some flash photogmphs appear red (Plate 5.3). Light from the flashbulb is focused on the retina by the lenses of the eye. and some of this light is retroreflected- a phenome­ non similar to dew heiligenschein (Sec. 3.4B), only here the reflection occurs on the retina, inside the eye­ ball, rather than on the blade of grass. outside the dew drop. Be­ cause the light is reflected back along the direction from which it came, the effect is greatest with cameras that have the flashbulb near the camera lens. Nocturnal animals like cats need to respond to low light levels. Rather than a choroid. then, a cat's eye has a reflective tapetum luci­ dum. * The light that misses the photoreceptors on the first pass is reflected by the tapetum and gets a second chance to be absorbed. This reflection slightly defocuses the light, so the resultant image is de­ graded , blurred . Again we see a trade-off of resolution for light sen­ sitivity. The light retroreflected by the cat's yellowish tapetum isn't completely absorbed on its second pass through the retina. Some emerges from the eye, giving the cat's eyes the often sinister yellow­ ish glow that your headlights some­ times reveal . Notice in Figure 5.10 that the photoreceptors are behind the neural network in the plexiform layer. It is as if the retina (film) were in the eye backward! In normal film ,

*Latin, luminous carpet. Crystals containing zinc cause the reflectivity.

the antihalation backing and the emulSion are usually kept separate so the antihalation backing can be removed easily during development. The photoreceptors of the retina. however, are living cells , which need the nourishment provided by the blood vessels in the choroid. Al­ though the pleXiform layer in front of the photoreceptors absorbs some light, this is minimized since its cells are ' transparent and require only a thin network of capillaries. In the critical, foveal region, there are no capillaries (see the TRY IT) and the plexiform layer is thinner. (Since they live at the bottom of the sea. where light is scarce, cephalo­ pods-octopus, sqUid, cuttlefish­ have their light-sensitive receptors in the front of their retinas .) Another reason for having the re­ ceptors next to the "antihalation backing" is to diminish the chance of reflections from the intervening material, which would cause a loss of sharpness. For this reason, some special films have the antihalation backing between the emulsion and the base.

TRY IT FOR SECTION 5.3

Seeing blood vessels, capillaries, and cells in the retina Hold a small flashlight just above your closed eyelid and move the light back and forth slightly. The larger vessels on your retina ' s surface become visible as they cast a moving shadow on your retina. They appear like a fine network of rivers and tributaries. Notice that they lead away from the blind spot (as in Plate 5.1) . Also note that no vessels cover your fovea, at the center of your field of view . john Uri Lloyd, in his book Etidorhpa; or the End of the Earth describes a similar way to see these ves sels using a candle: Placing himselj bejore the sashless window oj the cabin, which opening appeared as a black space pictured against the night, the sage took the candle in his right hand, holding it so that thejlame was just below the tip oj his nose. and about six tnchesjrom hisjace. Thenjacing the open window he turned the pupils oj his eyes upward, seeming tojix his gaze on the upper part oj the open window space .

and then he slowly moved the candle transversely , backward andjorward. across. injront oj hisjace, keeptng it in such positton that thejlickering jlame made a paraUelline with his eyes, and asjust remarked, about six inchesjrom hisjace, andjust below the tip oj his nose. . . . "Try jor yourselj," quietly said my gUide. Placing myselj in the posttton deSignated, I repeated the maneuver, when slowly a shadowy something seemed to be evolved out oj the blank space behind me. It seemed to be as a gray veiZ , or like a corrugated sheet as thin as gauze, which as I gazed upon it and discovered its outline, became more apparent and real. Soon the convolutions assumed a more decided jorm. the gray matter was visible. fi11ed with venations, first gray and then red , and as I becamejamiliar with the sight, suddenly the convoluttons oj a brain in a11 tts exactness , with a network oj red blood venations, burst into existence. I beheld a brain, a brain, a living brain, my own bra i n, and as an uncanny sensation possessed me I shudderingly stopped the motion oj the candle, and in an instant the shadowyfigure disappeared. Lloyd' s character, of course, can only be said to see his brain in t"e sense that, as Aristotle said, " The eye is __ '1 off-shoot of the brain. " To see the smaller, finer capillaries of your retina , close your left eye, hold a pinhole very close to your right eye, and through the hole view a uniform bright area, such as a well-lit unlined page or a cloudless sky. Now, without moving your eye, jiggle the pinhole with a small circular motion. The small squiggly lines that appear on the uniform bright area are the shadows of small blood vessels on your retina. Convince yourself that there are no vessels in front of your fovea. You do not need any devices, just patience and care, to see the effects of blood cells nourishing your retin a (these are not the same as the dead blood cells in the humors, the floaters). Simply gaze at the brig ht cloudless sky. If you 're observant, you 'll see tiny white dots wandering across your field of view at points away from the fovea . These are the effects of single blood cells as they pass through the small capillaries you saw earlier. (You won 't see these cells on your fovea, which is nourished at the rear.) Notice that the motion of the cells is greatest a short time after each heart beat. This is particularly evident a fter exertion, such as jogging, when the heart is pumping very hard.

5.3 THE RETINA

153 cones (sensitive to high light levels) and 120 million rods (sensitive to low light levels) in each retina, in an area of 5 cm 2 • If you could image it sharply, a baseball nearly one mile away would produce an image that would cover just one cone. Figure 5.12 shows that the distribution of these cells across the retina is not uniform. The relative numbers of rods and cones in the eyes of other animals differ from ours, depending on the animal's particular life style. Thus, in nocturnal animals, such as cats and rabbits, the photosensitive cells are all or mostly the highly sensitive rods. allowing the animal to see un­ der low, night, light levels (though they cannot distinguish colors). Some diurnal animals, such as pi­ geons, turtles, and mongooses, have only cones.

A. The rods a nd cones The heart of the retina is the light­ sensitive photoreceptor layer (Fig . 5 . 11). There are about 7 million

FIG URE 5.11

FIGURE 5.12

Electron micrograph of the back of a retina. The photoreceptors face you, and the light would strike them from the far side. The rods are big and blunt; the cones are smaller and pointy. Each cone is about 1 fLm across, and each rod is about 5 fLm across.

Distribution of rods and cones along the equator of an eyeball. The fovea has no rods but many cones. Rods predominate in the periphery. The blind spot has no photoreceptors. Inset shows left eye viewed from above. (Check your answer to the PONDER of Sec. 5.2.) Visual axis

Bl ind spot 780,000

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760,000

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740,000

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FIGURE 5.1 5

(a) Latency. (b) Persistence of response.

tensities to to second at high inten­ sities (Sec . 7 .7A). This is suffiCient, combined with the brain's ability to interpret motion, to allow us to see many moving objects clearly. (Not all, by any means, as you know if you've ever tried to read the label on a rotating phonograph record. ) In a camera, of course, the exposure time not only helps stop the action, it also limits the amount of light during an exposure. Good cameras have exposure times ranging over a factor of a thousand, much greater than the factor of two in the persis­ tence time of the human eye. Thus, this mechanism, like the iris, does not give the eye much control over the range of light intensities we ex­ perience in the world . In the next section we 'll see how the eye does allow for the large range of light in­ tensities.

occurs in your eye-your retina au­ tomatically changes its sensitivity (Its speed). In a sense, the retina contains two types of film ; one is made up of the sys tern of cones and the other, the system of rods. The former is analogous to a slow, fine-grain, color film while the latter is analo­ gous to a fast, coarse-grain, black and white film. The cones are sen­ sitive to high levels of light----pho­ topic' conditions-such as you 're experiencing now. The rod system is sensitive to low light levels­ scotopict conditions-such as o ~­ cur at night in the forest. The rods " turn off' under photopic condi­ tions; conversely, the cones do not respond under scotopic conditions. Your resolution is not uniform across the retina. The cones are densely packed in the fovea to help produce your high resolution and color vision there. Away from the fovea there are few cones, so your peripheral color viSion is quite poor. Cone density is not the com­ plete story, though; your resolution is determined by how much infor­ mation gets to your brain via the fi­ bers of the optic nerve. Typically, just a few cones in the fovea are connected to the same optic nerve fiber, while several thousand rods in the periphery of the retina may be funneled into one fiber. This funneling implies that the re tina must, in some way, process th e im­

D. Sensitivity In Section 5.2C we found that the action of your iris (stop) is not suf­ fiCient to compensate for the enor­ mous range of light intensities you experience in the world. Likewise, we saw that the processing time (Sec. 5.3C) can only vary by a factor of two and thus is also inadequate for this purpose . What else is there? In the camera, you can use films of different speed-a fast film for dark conditions, a slower film when enough light is present. Changing a camera's film every time you walk from the noonday sun into a base­ ment may be cumbersome and awk­ ward, but in fact such a process

'Greek phos, light. tGreek skotos, darkness.

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CHAPTER 5, THE HUMAN EYE AND VISION-I , PRODUCING THE IMAGE

156

age-less funneling in the fovea re­ sulting in more fine de tails of the image being sent to the brain from this central region (see the TRY IT). The rod system is sensitive under scotopic conditions , but alone it cann ot mediate color perceptions. That is why everything appears black, white, and gray in low light levels-as John Heywood wrote , "When all candles bee out, all cats be grey. " Lacking rods, your fovea doesn't work under scotopic condi­ tions . To detect a dim spot of light, such as a dim star at night. you avert your vision slightly-look to the side of the star a bit-so that its image falls off the fovea and onto a region of many rods. Because many rods are "wired together" into a sin­ gle optic nerve fiber, their sensitiv­ ity to low light levels is increased . Thus , as in film, the faster the "film" speed, the coarser the grain. The two systems, rods and cones, provide two different "film speeds" to deal wi th different levels of light. Even within just one system, how­ ever , the eye adjusts its sensitivity as the light level varies. The chang­ ing of sensitivity of the retina in re­ sponse to overall light level is called adaptation and can be illustrated by considering what you see when you walk from the midday sun into a dark theater. At first, it's too dark to see anything. You may acciden­ tally bump into chairs or sit on an old lady's lap. Bit by bit, though, your retinas adapt to the lower light levels (scotopiC conditions) and you can begin to see the people around you . Even tually you can make out your surroundings, but they appear only in black, white, and grays. You don't see much fine detail-you can't read your program. Through­ out the period of adaptation, a con­ stant intenSity source, sayan EXIT sign, appears brighter and brighter. Let's be more speCific. As a mea­ sure of the sensitivity of the retina, we can plot the intensity of a spot of light that is just barely detecta­ ble. This intenSity is called the threshold of d etec tio n, or simply, threshold. In Figure 5.16, we plot your threshold as a function of the time you are in a dark room. The vertical axis denotes the intenSity of

about 30 minutes. After a very long time in the dark, you can detect a light intensity equal to that of a candle viewed from 10 miles away! During World War II , blackouts re­ quired that all lights be extin­ guished; even a single lit Cigarette could be seen by enemy pilots .

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10 75 20 25 Time in dark (minutes)

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If we repeat this experiment using a small FIGURE 5.16

Dark adaptation. The threshold of detection (log scale) versus time in the dark. The first section (photopiC) is due to the cones. You sense color at these thresholds. The second section (scotopic) is due to the rods . You see the world as black, white , and gray at these thresholds. Details of this curve will vary, depending upon the specific conditions and the individual subject.

the light at threshold . Values at the top of the graph mean your thresh­ old is high, your sensitivity poor, as the source must be intense for you to see it. Conversely, values at the bottom of the graph mean that you are quite sensitive. We use a loga­ rithmic scale (as in Fig. 4.42) be­ cause the range of your sensitivity is so great . The horizontal axis is the time in the dark, measured in minutes. The range of times of in­ terest is not great, so we can use a regular (linear) scale for the time axis. On the left, the graph shows that just after you come into a dark room from a bright area, an intense light is needed for you to detect it. During the next few minutes your retina becomes more sensitive-the light intenSity at threshold de­ creases , at first rapidly, and then more slowly. When shown the light, you can tell its color, a sure sign that your cones are still operating. About 5 to 10 minutes after you en­ ter the dark room, the curve drops rapidly again, as you are rapidly getting more sensitive at this time. After this kink, you cannot tell the color of the light at threshold-you C\re now using rods only. (Of course , . f you look at brighter light, such as from the stage , your cones respond and you can distinguish the colors.) The curve becomes nearly flat at

light source and its light falls only on the fovea, no kink is found. Why? If the light falls only on the periphery, the threshold is very high until around 7 minutes, and then it joins the curve in the figure. Why?

When you go from a dark room to the bright outdoors , the process Is reversed; at first you are "blinded" by the light. (This phenomenon is sometimes used in the theater wi th great dramatic effect. The opening scene of Mussorgsky's opera B oris Godunou is played in dim llght, making the following scene-Bor­ is's coronation-appear all the more dazzling.) Thus, when you enter a dark room, your cone sensitivity begins to increase. When it can increase no more (the leveling off at around 5 minutes in Fig. 5.16). you switch to the rods. If necessary, their sensi­ tivity will also increase . Using these two different systems, each with ad­ justable sensitivity, you autom ati­ cally and continuously adjust your "film speed" until you have the ap­ propriate one for the available light. While this is a slow mechanism compared to the response of th e iriS or the processing time, it has a much greater range available . In a camera, when you switch to faster film, you usually have to sacrifice grain size, the sharpness of the final picture. Further, the fastest films are black and white. Similarly, in the eye, the switch to rods gives us a coarser-grained, less-detailed , black and white view of the world. Whenever necessary, you trade sharpness of vision for sensitivity to light. Whenever possible, you trade back. The switch from cones to rods during dark adaptation has anoth er consequence. Figure 5 . 17 shows

5.3 THE RETINA

157

, \

\

\ "

Rods

" ,/ '

350

400

....

,-" 500

600 700 800

Wavelength {nm}

FIG URE 5.77 Relative thresholds of the rod and cone systems versus wavelength of light at some late stage of dark adaptati o n. The rod system is, overall, more sensitive than the cone system and is mo st sensitive at about 505 nm. The cone system is most sensitive at about 555 nm.

the relative thresholds for the rod and the cone systems versus wave­ length of light. We see that, overall , t h e rod s are more sensitive (have a lower threshold) than the cones, but ther e is another difference ; the rods are m ost sensitive at the short wavelength end of the spectnIm, the blue, while the c nes are most sensitive at the long wavelength end, the red. Thus, at light levels high enough that the cones are op­ erating, a given red object may ap­ pear brigh ter than a blue object. On the other hand, at light levels so low that only the rods operate, the same blue object may appear brighter than the red one. This change from the red appearing brighter at high light levels to the b lue appearing brighter at low light levels is called the Purkinje shift. * You can easily experience th is effect using Plate 5 .2 . Notice in bright light that the red region appears brighter than the blue; turn off the lights and wait about 10 minutes in the dim room and notice that the blue re­ gion appears brighter than the red. T his effect is utilized in situ­ ations where one must rapidly change from photopic vision to sco­ 'After Johannes Purkinje (1789- 1869l. who first measured the dark adaptation curve (Fig. 5.16).

topic vision. For instance, astrono­ mers may need their precise cone vision for such work as checking gauges, and shortly thereafter need their rod vision for work at the tel­ escope. To allow for this rapid change, they use red lights to illu­ minate the observatory office build­ ing. The red light stimulates the cones preferentially, leaving the rods only slightly stimulated. In this way, the rods can stay more dark adapted while the cones are used for reading the gauges. The adaptation of each of your eyes is independent of the other; one eye can be operating scotopi­ cally, while the other is operating photopically. When you're wakened in the middle of the night, keep one eye shut while you turn on the lights. Later, when you turn off the lights, open that eye (which is still dark adapted), and you can see clearly and avoid stumbling over the junk on your floor.

TR Y IT FOR SECTION 5.JD

Foveal versus peripheral viewing The fovea is responsible for the sharp vision you need to read this book as weir as for color vision. The peripheral parts of the retina provide neither of these. To demonstrate these facts, stare fixedly at one letter in the center of this page from about 25 em away. Without moving your eye, try to read some other part of the page. If you don't move your eye, you may be surprised at the limited region of the page over which you can make out the words. Still keeping your eye fixed, ask a friend to hold up some fingers to your side and see if you can count them . These experiences should convince you that your sharp vision is only from the central, foveal, part of your retina. To demonstrate the lack of color discrimination in the peripheral parts of your retina, again stare at one point. Have your friend hold a few colored pens or swatches of colored paper to your side while you attempt to judge the colors. Can you tell which color is which? On the other hand, if your friend holds up two pieces of paper, one black and one white , can you distinguish them? Th at is, can you distinguish the relative brightness of the light coming to the periphery of your retina?

SUMMARY

The compound lens (cornea p lus eyelens) forms an image on the "film" (retina) of the eye, which contains a nonuniform distribution of the light-sensitive rods and c ones. The diaphragm (iris ) closes quickly to reduce aberrations and increase depth of field when enough light is present. The retina consists of a small area for precise vision (the fovea, containing cones), a broad area for more sensitive nigh t vision (the peripheral parts of t h e retina, containing rods), and a re­ gion of no receptors (the blind spot, where the nerve cells leave the retina). This structure necessitates moving your eye as your view a scene (sc a nning). To change the focus of the eye as you look from distant to near objects, you accom­ modate (change the focal length of the eyelens). The fovea is responsi­ ble for the fine vision and . bein g near the axis of the eye. is least af­ fected by aberrations. The brain's ability to learn is also crucial in overcoming aberrations. The retina contains a light-sensi­ tive layer of photoreceptors, a p lexiform layer of nerve cells to carry the signals to the brain, and a dark c horoid layer. which serves as an antihalation backing. In the photoreceptors. light affects the photochemicals, causing an elec­ trical signal to pass along the cell to the synapse, which links t h e pho­ toreceptor to subsequent nerve cells . The receptors take some time to start responding (latency). Most moving objects don't look blurre d because, although the receptors keep responding (p ersistence of reponse), they do so for only a cer­ tain time. The retina's "speed" (sen­ sitivity) is gradually changed auto­ m a tically, depending on the overall light level. This process of adapta­ tion changes the "film" fro m t h e fine-grained. color vision of th e fov­ eal cones in bright light (p hotopic vision) to the sensitive. coar s e­ grained. black and white scotopic vision of the peripheral rods in dim light.

CHAPTER 5, THE HUMAN EYE AND VISION-I, PRODUCING THE IMAGE

158 PROBLEMS

Pl Compare and contrast the eye with the

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basic camera. What part of the eye corresponds to each major component in 1he camera? What is accommodation and how is it achieved? What are the layers in the retina (the " film " )? Compare and contrast film and the human retina . Di scuss how the eye and brain deal with the problems of aberrations. Pay particular attention to chromatic and spherical aberration, but discuss them all. Why do humans have choroids while nocturnal animals have tapeta? Explain in a brief sentence or two why a " black light" looks fuzzy. (a) What is the blind spot? (b) You don't "see" it as a dark spot in your field of view. What do you see? For example, draw a straight line. Draw another straight line parallel to the first, but shifted over by 2 mm, beginning even with the point where the first ends and continuing on. The two should look like one long line with a break in it. " Look" at the break with your blind spot. Can you tell there is a break in the line, or does it look like one long straight line? Try to make some other pattern which, when "looked" at with your blind spot, creates some interesting effect. In The Divine Com edy, Dante says, " a sudden flash blinds the eyes/so that it deprives them/of the sight of the clearest objects. ." Explain the phenomenon that Dante is talking about. What is the Stiles-Crawford effect? What is thought to be the mechanism for the effect? What benefit might this effect bring to human vision? In The Girl in a Swing, Richard Adams writes: "Look, everything's silver out

there-the roses, the lupins, everything! Have you noticed, they've got no colour at all by moonlight?" Explain the phenomenon described . Pl1 During the day, a certain red geranium appears brighter than a blue violet next to it. At dusk, though, the situation is reversed; the violet now appears brighter. Why? P12 Isaac Newton, fascinated by the sun, viewed it through a focusing lens: In a few hours I had brought my eyes to such a pass that I could look upon no bright object with neither eye but I saw the Sun before me, so that I durst neither write nor read but to recover the use of my eyes shut myself up in

my chamber made dark three days together and used all means to divert my imagination from the Sun. For if I thought upon him I presently saw his picture though I was in the dark. Explain what Newton's problem was and why it had come about.

HARDER PROBLEMS PHI Discuss the trade-off between sharpness of vision and sensitivity to low light levels in the eye. What mechanisms are responsible for this trade-off? PH2 Suppose the threshold of detection experiment was done using a small light source. Sketch the analogous ' curve to that of Figure 5.16 for the case where the light from the test source falls only on: (a) the fovea, (b) the periphery of the retina. PH3 Suppose the curve of Figure 5.16 had been obtained by measuring the threshold of detection of a red light (650 nm). What would the curve look like? (See Fig. 5.17.) PH4 In "Stanzas on Death ," Jean de Sponde writes:

My eyes, no longer cast your dazzled gaze Upon the sparkling beams of fiery life; Envelop, cloak yourself in darkness, eyes: Your customary keenness not to dim, For I shall make you see yet brighter lights, But leaving night you shall see all the brighter. What phenomenon of human vision is de Sponde referring to? PHS Hold a pinhole in a piece of aluminum foil about 3 to 5 cm from your eye and look through it at a bright light (e.g. , a light bulb t m away) .. Take an ordinary straight pin, stick it in an eraser on top of a pencil, and put it between your eye and the pinhole, moving the pin up and down so you can see the pinhead crossing in front of the pinhole . (a) Describe what you see . (b) Draw a picture showing the light bulb, the pinhole, the pin, your eye, and rays coming through the pinhole from the top and the bottom of the light bulb. Use this picture to explain what you see. PH6 Hold an 8-cm object 30 cm from your eye. Notice that you must move your eye to view the entire object carefully. How could you use this observation to estimate the size of your fovea? Explain.

MATHEMATICAL PROBLEMS PMI If the (compound) lens of the human eye were taken out and examined, it would have a focal length of about 16 mm. (In the eye, however, there is not air behind the lens, but rather the vitreous humor with an index of refraction of about 1, so the focal length of the lens in the eye is somewhat longer.) When dilated, the pupil diameter is about 4 mm. What is the speed of this lens (in air)?