Ординатура / Офтальмология / Английские материалы / Relearning To See_Quackenbush_2000

When the retina is examined immediately after being in darkness, it has a reddish-pur- ple color. This "visual purple" is due to a pigment produced inside the rods called rhodopsin (from the Greek rhodon, meaning "rose," and opsis, meaning "sight").

Rhodopsin is produced continually by the reaction of the yellow-orange aldehyde retinal with one type of a colorless protein called opsin. Retinal is a modified form of Vitamin A, found in carrots and many other vegetables.

When bright light is absent, rhodopsin increases its concentration in the rods, giving the rods the ability to pick up extremely low levels of light. The higher the concentration of rhodopsin, the greater a rod's sensitivity to light.

When light hits a highly "rhodopsinenergized" rod, an electrical signal is sent through the retinal cells to the brain. But now the rhodopsin pigment is disassociated; the visual purple is "bleached out." This rod can no longer pick up low levels of light—at least temporarily. As long as the level of light remains low, rhodopsin is reformed at a greater rate than it is depleted, and the rod quickly regains its sensitivity to low levels of light.

Bright light, like daytime light, causes the rate of depletion of rhodopsin to be greater than the rate of formation. As a result, the rods lose their sensitivity to low levels of light.

So, the rods are like rechargeable batteries. In darkness, they become fully "charged"; but in bright light they become relatively "discharged."

Smoking often reduces nighttime vision. The rods need to be healthy to pick up low levels of light. Fortunately, much of nighttime vision can be restored when smoking is stopped.

Chapter Seventeen: The Retina

Excellent Night Vision

Due to the high concentration of rhodopsin at night, rods can pick up extremely low levels of light. When fully adapted to darkness, the rods are up to 30,000 times more sensitive to light than the cones are in daytime. (Cones have no sensitivity to extremely low levels of light.) A highly-sensitized rod can be "triggered" by a single photon—the smallest unit of light energy.

Steve Richards writes, "The normal human eye in good health is capable of detecting the Ught of a match on a clear, dark night at a distance of thirty miles!"2

From Brightness to Darkness, and Back

Wben a person goes from bright light into sudden darkness, the rods reach ~8o% of their low-level Ught sensitivity in fifteen minutes. Rods reach complete night adaptation in one hour.

When you wake up in the middle of the night, the rods have adapted to very low levels of light. You can see the objects in your room. But when you turn on a bright light (Ouch! The visual purple does not like bright light!), the visual purple is quickly lost, and along with it, low-light sensitivity. When the light is turned off, almost no objects can be seen in the room, and the process of night adaptation starts again.

Going to the movie theater in the daytime, when the movie has already begun, is an education in darkness adaptation. When the door to the theater closes behind you, you can see the movie screen and the aisle lights, but little else. The pupil enlarges and the rods begin adapting to the darkness. Gradually, you see

better and better, and soon enough, you can walk down the aisle. "No one is sitting in this chair. Oops! I guess I haven't adapted to darkness as much as I thought!"

By the end of the movie, before the lights are turned on, the theater seems brighter than when you first entered. You adapted to the darkness during the movie. If it is still day­ time when you leave, bright light hits the rhodopsin-sensitized rods and the visual pur­ ple is bleached away once again.

Nighttime Tip #1. If you have adapted to very low levels of light, look away from any sudden bright light. If you look toward the light, you will not be able to see as well, if at all, for a short period of time.

Nighttime Tip #2. If you are leaving a room in which you have adapted to the darkness, but plan to return very soon, before entering the bright room, cover one eye. In this way, the eye you cover will still be darknessadapted when you return to the dark room.

R O D S — O U R " M O V E M E N T D E T E C T O R S "

tralizing seems steady. Monitor flicker is dis­ cussed more in the Chapter 24, "Computers, TVs, and Movies."

Some people have retinas in which there are rods but no cones—a condition known as rod monochromacy.3 For this 0.003% of the population, the picture of the world has no detail and is completely colorless.

... A N D T H E N T H E R E W E R E CONES

ЛI ft

As animals evolved from the deep dark oceans to bright land, some of the rods evolved into cones. The high intensity of sur­ face light is utilized by the cones to pick up sharp detail and colors. Cones are found pri­ marily in diurnal (daytime) animals and humans. The majority of the seven million cones in the human retina are located in the fovea centralis.

The rods are excellent "movement detectors." Rods allow animals to detect and catch mov­ ing prey, and to escape moving predators. As discussed in Chapter 9, "The First Principle— Movement," the rods (and cones) are not designed to be stimulated with a constant, steady source of light. Light rays need to change their positions on the retina. As long as we are moving, light rays from stationary objects change their positions on the retina.

While working with some older computer monitors, a "flicker" can be seen in the periph­ eral vision. This is due to the rods picking up the slower "refresh" rate of the screen. Since the cones do not pick up movement as well as the rods, the point at which you are cen-

T H R E E T Y P E S O F C O N E S

See Plate 33: Daytime Cones Sensitivity Chart.

There are three types of cones in the retina:

1."Blue" cones are most sensitive to vio­ let and blue light.

2."Green" cones are most sensitive to green and yellow light.

3."Red" cones are most sensitive to green, yellow, and red light.

These three types of cones result from reti­ nal combining with three different types of opsins inside the cones. "Blue" cones have a higher concentration of "blue-opsins"; "green" cones have higher concentration of

"green-opsins"; and "red" cones have a higher concentration of "red-opsins."

By combining the sensitivities of the three cones in Plate 33, we can see that the greatest sensitivity is in the yellow-green region. Note the logarithmic vertical scale.

cones have no sensitivity. The cones need at least a medium intensity of light to be activated.

Artificial lights—for example, a flashlight— used at nighttime are designed with enough intensity to activate the cones. We can then see detail and colors.

TRI-CHROMATIC VISION—

OUR NATURAL R G B M O N I T O R

See Plate 34: The Eye—Our Natural RGB

Monitor.

Most computer displays (CRTs, VDTs) and TVs are "RGB" monitors By combining red, green, and blue lights on a screen in different amounts, all colors can be created.

The full range of colors perceived by the brain is a result of different amounts of the blue, green, and red cones being stimulated by a particular object.

For example, light waves from an "orange" ball would stimulate more "red" cones than "green" cones, and none of the "blue" cones. The brain interprets this mixture of red and green signals as orange. Similarly, a blue object would stimulate more "blue" cones than "green" cones, and none of the "red" cones.

The three different types of cones—red, green, and blue—allow us to see all the colors of a rainbow. This is known as tri-chro- matic vision. Cones can also pick up white, which is the combination of all colors.

CONES NEED MEDIUM-BRIGHT INTENSITY

Plate S3 shows the sensitivity of the three types of cones in bright light. When darkness approaches, the cones begin losing their sensitivity to light. In "true" nighttime vision, where the intensity of light is very low, the

Returning to the film analogy, the cones function somewhat like low-speed color film, like ISO/ASA 100. Low-speed film has a finer grain than high-speed film. It produces good definition (sharpness), higher contrast, and strong color saturation. However, low-speed film is less sensitive to light than high-speed film.

M O S T C O L O R B L I N D PEOPLE SE E COLORS

Many people think that people who are "colorblind" cannot see colors. However, most colorblind people have lost only part of their color vision. Usually, only one or two types of cones are either absent or not functioning normally. If the green cones are not functioning normally, a person will be deficient picking up green. However, red and blue and combinations of red and blue can still be seen.

Five to eight percent of men and 0.3-0.5% of women are colorblind. Although color blindness is considered to be hereditary, some natural vision students have improved their color blindness using the Bates method.

Clara Hackett, in her natural vision book Relax and See, presents some excellent activities for people who have color blindness She writes:

The absence of all appreciation of colours is very rare. In most cases, there is a lack of perception of red, green, and/or blue. Many colour-blind people can develop their colour sense.

... The techniques [described] are practical for self-instruction. If you are colourblind, there is a good chance that you can benefit from them. Essentially, they add up to a double learning process. You will learn basic facts about colours and practice identifying, matching, making and sorting colours. You will also practice some of the basic elements of good vision and these will help to simplify and speed the colour learning process.

A first essential is to achieve relaxation. Tension affects your mind and memory and may interfere, too, with "sighting" of colour. If previous attempts to learn colour were futile, tension may have been a factor.4

Syntonics, mentioned in the previous chapter, has also benefited some people who have color blindness.

A DIFFERENCE BETWEEN DAY AND NIGHT

According to historians, early Muslims determined the tegjnning of day when natural sunlight allowed them to see the colors of threads in a pile of mixed fibers. When the colors were not distinguishable anymore, it was the beginning of night.

See Plate 36: Cones and Rods Sensitivity-

Day and Night Cycle.

In the daytime, cones have good sensitivity to colors and detail. Rods are not as sensitive to light in the daytime because visual purple is absent. In true nighttime vision, the rods become extremely sensitive to low levels of light, and the cones drop to zero sensi- tivity—in other words, the cones do not function.

Of course, if a light source has enough intensity in the night, it can activate the cones, and therefore, color and detail are seen. Notice that detail can be seen on the moon at night. There is sufficient intensity of Ught from the moon to activate the cones. Still, true nighttime vision is primarily rod vision.

DIFFERENT DENSITY DISTRIBUTIONS

Understanding of density (concentration) distribution of cones and rods in the retina helps us use our central and peripheral vision correctly.

See Plate 37: Measuring Density Distributions of Cones and Rods.

The Vertical Density Graphs (discussed below) are based on measurements taken along, and above and below, the vertical line V1-V2. These graphs show the typical density distribution of the cones and rods along lines passing through the fovea, without the "distraction" of the optic disc.

The Horizontal Density Graphs (discussed below in the section "The Blind Spot—No Cones or Rods") are based on measurements taken along, and to the left and right of, the line H1-H2. Notice how this line passes through the optic disc (OD), where there are no cones or rods.

CONE DENSITY DISTRIBUTIO N

Maximum Cone Density at the Fovea Centralis

See Plate 28: Retina Cross-Sections, b-3, Plate 30; Retina (1), and Plate 37: Measuring Density Distributions of Cones and Rods. See also Plate 38: Cones Vertical Density Graph (Vi-> V2) and Plate 39: Cones 3-D Density Model

(Side View).

The macula lutea (from the Latin macula, meaning "spot," and lutea, meaning "yel- low"—literally, "yellow spot") is -1.25 mm (50) in diameter and contains a very high concentration of cones.

The fovea centralis (from the Latin fovea, meaning "pit," and centralis, meaning "cen- tral"—literally, "the central pit") is a very small depression located in the center of the macula. The fovea is about Vs the size of the macula, or -0.25 mm (i°) in diameter. The fovea consists almost exclusively of cones. There are only cones exactly in the center of the fovea—no rods.

Plate 38 shows the density of cones as measured vertically along the retina.

Density, in this case, refers to the number of light receptors per square millimeter (mm2). 1 mm « V 2 5 " . Density can also be considered the concentration of light receptors. Generally, the higher the density, the better we can see.

Moving from the peripheral part of the

retina into the macula, the density of cones increases exponentially. Cone density increases to a maximum of ~i50,ooo/mm2 exactly in the center of the fovea.

The fovea is literally a pit in the retina. As mentioned at the beginning of this chapter, there is an exception to the light rays needing to penetrate eight retinal layers to reach the cones and rods. At the fovea, some of the top retinal layers, particularly the ganglion and bipolar cells, are compressed and moved off to the side. Additionally, the cones "reach" slightly upward toward the top of the retina. With less distance to travel, Ught rays can hit the foveal cones without scattering the Ught as much.

These facts result in our sharpest vision— by far—being at a pinpoint in the center of the visual field. The area of peak concentration of cones in the center of the fovea is so small, the eyes need to shift from one dot of a colon (:) to the other in order to see each dot distinctly!

Charles H. May, in Diseases of the Eyes, writes:

Cones are concerned with visual acuity and color discrimination at high intensities of illumination (photoptic vision); rods are responsible for vision at low degrees of illumination (scotoptic vision) when sight is more effective in the periphery of the retina and is colorless. When the image of an object falls upon the macula, there is distinct vision; when it falls upon any other part of the retina, there is indistinct vision. Two points give rise to separate visual impressions when their images are at least 0.002 mm. apart, since this represents the diameter of the cones at the fovea. In other words, to be seen distinctly, two objects must subtend a visual angle of one minute or more.5

One minute of an arc is one-sixtieth of i°, or 0.016° (A circle is 3600 ; 90° forms a right angle.) As you can see, so-to-speak, the area of sharpest vision is extremely small.

One way to remember the relationship of cones to other aspects of our central vision is to think of all the "C" words: "Cones see Clearly and Colorfully only in the Center at the fovea Centralis; Chickens have all Cones!"

The cones are concentrated in the fovea, and the rest of the retina has a higher proportion of rods which only provide monochrome [black and white} vision. The illusion of full colour outside the central image is provided by the brain. This can be demonstrated by moving a previously unseen object into a person's field of view. The person is unable to identify the colour until the object is close to their main image area.6

Minimal Cone Density in the Periphery

The density of cones, along with clarity and color perception, decreases dramatically moving away from the fovea. The density of cones drops to only 3% of then maximum, at a distance of only 70 from the center of the fovea. The density of cones is even lower outside the macula, and drops to zero in the far peripheral parts of the retina.

Not only does the cone density drop dramatically in the peripheral parts of the retina, the eight upper layers of the retina he above those few peripheral cones. As a consequence, peripheral cone perception ranges from verypoor to none. At a distance of 70° into the peripheral retina, acuity is only 1% of the central vision; this is vision. Color is imperceptible in the far parts of the peripheral vision.

In Plate 39, the (minimal) peripheral cones are darkened to indicate they are "buried," like the rods, under eight retinal layers.

The cones do pick up some color in the peripheral vision, especially close to the central vision, but it is "diluted" compared to color in the central vision.

Zoologist John Downer, in his book Super- sense: Perception in the Animal World, writes:

EXPERIENCE C O N E DENSITY

DISTRIBUTION

As discussed in Chapter 10, "The Second Prin- ciple—Centralization," it is impossible to see clearly except in the center. People who have blurry vision attempt, usually subconsciously, to see eveiything clearly at the same moment. This is called diffusion. The attempt to do the impossible, Bates said, is a strain, and lowers sight.

In order to see clearly, a person needs to have his visual attention at the place within the picture where it is clear, and the only place the picture is clear is exactly in the center.

^ E X P E R I E N C E C O N E D E N S I T Y D I S T R I B U T I O N

See Plate 8: Centralizing—The Two Pencils.

Take two different colored pencils, for example, yellow and green. Hold the bottoms of the pencils, with the eraser ends positioned on top. Hold the pencils vertically out in front of you about one foot away. Separate them horizontally about 16" away from each other.

Sketch the yellow pencil while wiggling the peripheral green pencil. The cones in fovea pick up sharp detail and bright color on the yellow pencil. The green pencil, if held far enough out in the periphery, will appear gray

and "unclear." No peeking at the green pencil! There are so few, "buried" cones in the peripheral part of the retina, they are of no value in picking up the detail and color of the "green" pencil. In fact, if you had not known beforehand the second pencil was green, you most likely would not be able to identify its color. On the other hand, the movement of the peripheral "gray" pencil is picked up well

by the rods.

Now sketch the green pencil. A very dif­ ferent experience! Now the cones in the fovea pick up the sharp detail and bright green color of the green pencil—right where you cen­ tralize. The "yellow" pencil is now "colorless" and indistinct. Wiggle the yellow pencil. Once again, the rods pick up the peripheral "gray" pencil's movement very well.

ROD DENSITY D I S T R I B U T I O N

Maximum Rod Density in the Periphery

See Plate 28: Retina Cross-Sections, a-3, Plate 30: Retina (1), and Plate зу: Measuring Den­ sity Distributions of Cones and Rods. See also Plate 40: Rods Vertical Density Graph (Vi- V2), and Plate 41: Rods 3-D Density Model

(Side View).

The rods are not distributed evenly on the retina. The maximum density of rods is ~i6o,ooo/mm2, and they are located in a 3600 circle around, and -18° away from the fovea. The density of rods being "maximized" around the fovea allows us to have our best nighttime vision (only 20/400 at best) close to the center of our visual field. This is impor­ tant, because in very low levels of light, we have no vision exactly in the center of our visual field.This is yet another remarkable

Chapter S e v e n t e e n : The Retina

design of nature—giving us our best night vision close to where we have no (central) vision.

Note: The loss of central vision described here is not the same as the "blind spot" caused by the optic nerve. This is discussed further below.

Moving toward the far periphery (from the circle of maximum density), the density of rods slowly diminishes to about one-quarter of its maximum. The furthest parts of the visual por­ tion of the retina contain only rods, no cones

Zero Central Rods

Moving toward the fovea (from the circle of maximum density), the density of rods quickly drops to zero. There are no rods located exactly in the center of the fovea, only cones. In fact, by the rods dropping to zero density, the cones can rise to their maximum "pure" density in the center of the fovea.

Since there are no rods in the center of the fovea centralis, and since the cones do not function in very low levels of Ught, aU humans are correctly "blind" in the center of the visual field in true nighttime vision. True nighttime vision is any situation in which the intensity of light is sufficient to activate the rods, but not the cones.

Fortunately for our nighttime vision, the area of zero rods in the fovea is very small. This means the point of interest must be very smaU (and dim) for it to disappear. The corol­ lary to this is—if the tiny central point is seen at any time, that point must be cone vision, and detail and color can be seen at that point.

If an object is very dim, but also large, the center of that object will disappear, but the peripheral parts of that object will be picked up by the rods.

^ E X P E R I E N C E R O D D E N S I T Y D I S T R I B U T I O N

Part A: At nighttime, sketch a star in the sky. Notice there seems to be a "ring" of bright stars ~i8° away from that star, all the way around it in a circle. This effect is due to the "ring" of high-density rods located ~i8° around the fovea.

If the star you are interested in is dim enough, and if you are centralizing on it, it will disappear! This is because the cones cannot pick up very low levels of Ught and there are no rods in the center of the fovea.

Part B: In the dark, find a watch or other object which has small, dim, fluorescent dots. If you sketch one dot directly, it will "disappear." When you shift away from the dot, it "fights up" in your peripheral vision.

These are excellent ways to study rod vision. The student can determine how well she is centralizing by watching the tiny dim spot disappear.

The "disappearing act" of the tiny, dim star is experienced in darkness in one of the natural vision classes. This is one of the students' favorite classes.

If you were an astronomer, how would you ever see a tiny dim star? One possibility would be to use a powerful telescope, so that a dim star appears brighter. The star might then have enough intensity of light to activate the cones, and could then be picked up with the central vision. What if the star was still too dim, even in the telescope? You could locate the star in the peripheral vision, and then take a picture of the star. The film will register the star's light, but only if the film's "light receptors" are sensitive enough. After the picture is developed, you still need to put enough light on it to finally see the star clearly.

Many telescopes used for night viewing are a combination of two telescopes. The smaller, low-power telescope, called the finder, attaches to the side of the large telescope.The finder, analogous to our peripheral rods, picks up a larger but less detailed area in the night sky to "find" planets and stars.

R.L. Gregory writes in Eye and Brain—

The Psychology of Seeing:

Astronomers "look off' the fovea when they wish to detect very faint stars so that the image falls on a region of the retina rich in rods.7

Do not diffuse, even in true nighttime vision. Centralizing is an important mental function and should be practiced at all times In true nighttime vision, with only 20/400 vision, continue to centralize and shift your attention. Pick up your peripheral vision similar to the way you do in the daytime— "peripherally."

Being diurnal creatures, the human visual system is not designed primarily for night vision. If you are in very low-level light situations and need to see clearly, use a flashlight to activate the cones.

D I F F E R E N T N E T W O R K I N G

A close look at the middle layers in Plate 28: Retina Cross-Sections, reveals a difference in how the cone and rod signals are processed.

Each cone is connected to its own bipolar cell in the inner nuclear layer. This one-to-one connection between the cones and the bipolar cells is one reason we pick up a sharper image with the cones. This one-to-one cone connection is not maintained all the way to the brain, however. There is some mixing of cone signals in the inner plexiform layer, where several cones can connect to one ganglion cell.

Unlike the cones, several rods are connected to one bipolar cell, Additionally, the rods are connected by horizontal cells, located just above the outer plexiform layer. This grouped arrangement is called association cells and allows for better sensitivity to light at night and better movement perception by the rods; a disadvantage of association cells is a less distinct image.

This "tree branch" arrangement of rods and bipolar cells is one reason objects appear larger at nighttime compared to daytime. If only one rod is struck by a Ught ray, the brain interprets it as if an entire group of rods has been struck.

There are 137 million cones and rods, but only one miuion nerve fibers travel from the retina to the brain.

P U T T I NG I T A L L T O G E T H E R

In medium to high levels of light, the cones pick up sharp detail and colors, but only in the central vision. The rods are capable of picking up very low levels of grays in darkness, and are excellent at movement perception in the peripheral vision.

Plate 42: Cones and Rods Vertical Density Graph (V1-V2) is simply a combination of

Plate 38 and Plate 40. Plate 43: Cones and Rods 3-D Density Model (Side View) is simply a combination of Plate 39 and Plate 41.

Plate 35: Daytime Cones/Darkness-Adapted Rods Sensitivity Chart is a combination of Plate 32 and Plate 33. Plate 35: Daytime

Cones/Darkness-Adapted Rods Sensitivity

Chart shows how darkness-adapted rods pick up Ught much better than the bright lightadapted cones. (Of course both of these sensitivity charts cannot occur simultaneously, since both darkness and brightness cannot occur simultaneously.) The shift in maximum

sensitivity from -496 nm in darkness with the rods to -555 nm in bright light with the cones is known as the Purkinje shift. (Do you remember the images of Purkinje?)

See Plate 44: How We See.

a.In the daytime, we think we see like a camera. We think every part of the picture is equally sharp and colorful.

b.In daytime, humans only see clearly and most colorfully in the center due to the high density of cones in the fovea. The peripheral parts of the picture are much less clear and colorful. Also, the area of human vision is not a rectangle shape, but more an irregular, oval shape.

c.In true nighttime vision, humans are incapable of seeing a very smaU and dimly Ut central area. This is because there are no rods in the center of the fovea, and the cones are incapable of picking up very low levels of Ught. We are "blind" in the center. Notice how the rods have their greatest sensitivity in a circle -18° from the central point of interest.

The iris constricts in bright light, forming a smaU pupil, and expands in dim Ught, forming a large pupil. Not only does a large pupil size let in more light, but the size of the picture we see is larger by about 10%. For the right eye, most of this increase is toward the temple and slightly below eye level. For the left eye, the visual field increases to the left and slightly below eye level. Notice in Plate 44: How We See, c, the size of the picture on the lower left, left, and upper left side does not increase in nighttime. Why? Just cover your left eye to find out!

Notice the blind spot due to the optic nerve.

See also Plate 45: A Difference Between

Day and Night.

T H E B L I N D S P O T — N O C O N E S

O R R O D S

See Plate 30: Retina (1), Plate 31: Retina (2), and Plate 37: Measuring Density Distributions of Cones and Rods.

The optic disc is a depressed, light-pink area located inside the eye where the optic nerve joins the eyeball. The optic disc is ~2 mm in diameter and located -15° from the fovea, toward the middle of the head and slightly above the horizon. It has a slightly vertical oval shape.

See Plate 46: Cones 3-D Density Model (Top View) and Plate 47: Rods 3-D Density

Model (Top View). These illustrations show the blind spot at the optic disc, indicated by the small white oval located -15° to the right of the fovea centralis These models are of the right eye as if you were looking into the eyeball at the retina. The right side of the models are toward the nose and the left side of the models are toward the temple.

The white point in the center of Plate 47 indicates there are no rods in the center of the fovea. Notice the "ring" of high-density rods -18° away from and around the fovea. In Plate 46 the high density of cones in the macula is indicated by the many brightly colored, concentric circles in the center.

Plate 48: Cones and Rods Horizontal Density Graph (H1-H2) shows the cones (colored) and rods (black and white) densities along and beyond the (nearly) horizontal line HiH2 in Plate 37. Notice in Plate 48 how both the cone and rod density curves drop to zero density at the optic disc.

See Plate 49: The Blind Spots and Plate 51:

Binocular Vision. There are no cones or rods in the optic disc—only nerve fibers and the central retinal blood vessels. This area creates one "blind spot" for each eye. When see-

ing with only the right eye, there is an area located -15° to the right of and slightly below the point of centralization where there is no vision. The left eye's blind spot is located -15° to the left of and slightly below the point of centralization.

The area not seen with the blind spot increases the greater the distance an object is located from you. At a distance of ten inches from the eye, the area of the blind spot is about the size of a quarter; at one meter it is the size of an apple; at three meters, it is about the size of a basketball; and at twenty meters, the size of a horse!

Usually, a person is not aware of the blind spots because the area not seen by the right eye is picked up by the left eye, and vice versa. See Plate 49 and Plate 51. Another excellent design by nature, for if the optic disc were in the middle of the back of both eyes, we would have no central vision—night or day.

The brain tends to fill in the blind spot area to some extent. This can create a false sense of security for a person with sight in only one eye. It is especially important for people with diminished or lost sight in one eye to continue moving so that the area of the blind spot does not remain constant. Some people with sight in only one eye have been struck by an object coming toward them along the angle of the blind spot because they were not moving.

^ E X P E R I E N C E T H E B L I N D SPOT

Part A:

Cover your right eye. Hold this page with the words "Fovea Centralis" out in front of the left eye approximately eight inches away. While sketching the letter "v" in the word "Fovea" with your nose-pencil, slowly move this page closer to and farther away from your head. At a certain distance, the words