COMMON ABNORMALITIES OF VISION

The astonishingly wide variation of hues and tints and chromas is visible to the majority of human beings. There is a large class, however, whose visual range is much narrower. About three percent of the male population is known to be partially color blind, that is insensitive to certain visual stimuli. To such individuals blue and yellow appear as they do to persons with normal color vision; but they see both red and green as grey, greenish yellow and orange as greyish yellow, and bluish green and purple as greyish blue. Partial color blindness is hereditary and although it usually occurs only in males, it is transmitted by females. Thus a partially color blind man may have both sons and daughters who are color normal. All of the children of his sons may also have normal color vision, but the male offspring of his daughters will be partially color blind.

There are also a number of individuals who are totally color blind, who see no colors at all. Total color blindness is also hereditary and is also more prevalent among males than among females, but it is much rarer than partial color blindness and should be confused neither with that nor with true blindness. Blind persons see neither colors nor blacks nor whites nor greys. As Helmholtz put it they “see things in the same way in which we ourselves see what is behind our backs; that is to say, they do not see at all.” But totally color blind persons can see; in a dim light they can see exceptionally well. They can distinguish objects visually as well as persons with normal eyes, but for them these objects never possess any color. They always appear grey.

The two forms of color blindness seem more creditable when we realize that everyone is often either partially or totally color blind. All of us are partially color blind to objects which we see in indirect vision, or “out of the corner of the eye.” Many of us, of course, never realize this deficiency, because we usually pay very little attention to our indirect vision. If an object interests us we immediately turn our eyes toward it and focus them upon it and its colors become clear. But if you will hold an orange or a greenish blue pencil parallel to and about a foot away from your right ear while your eyes are steadily focused directly in front of you, and move the pencil gradually until it is directly in front of your nose, you will find that at the side of your eye the orange pencil looks only yellow and the greenish blue pencil only blue. The red and green do not appear until the pencil is almost within the focus of your eyes. It is only at the very center then that the eye is color normal. It is partially color blind at the sides. And it is always totally color blind, of course, in the dark.

If we wake up suddenly on a black night or go quickly from a brightly lighted room into one that is much darker, we at first seem to be quite blind—to be incapable of distinguishing anything. Gradually, however, our eyes become accustomed to the darkness—the psychologists say that they become dark adapted, and we can make out the outlines of familiar objects, can see that parts of the room are lighter and parts darker than others. But we cannot distinguish any colors. We do not even see any clear blacks or whites. Everything appears grey. And that is the way the whole world in daylight as well as in darkness looks to the totally color blind—as a series of darker or lighter greys.

The inability to distinguish the hues of colors is not the only thing which differentiates the way we see in the dark from the way we see in daylight. We have already noted the fact that under good illumination the center of the eye is the area of clearest vision. Those objects upon which our eyes are focused are the ones which appear clearest to us. Offhand we would suspect that the same principle held in the dark. Yet it is very easy to demonstrate that it does not. Some night when you are out of doors look up at the sky and select one small isolated star and remember carefully just where it is. Then try to focus your eye upon it. You will find that this is impossible. As soon as you stare directly at it it disappears, but reappears as soon as you search for it. This process can be continued indefinitely—a disappearing and reappearing which is so rapid that the star actually seems to twinkle. Whenever you look directly at it it is gone, but as soon as you shift your eyes ever so slightly it is back again. Carefully verified experiments which elaborate this simple experience have convinced psychologists of the fact that at night all eyes are completely blind at the center where in daylight they have the clearest vision. And totally color blind eyes show this same deficiency in daylight as well as in the dark. They twitch continually when their possessors attempt to read.

So much for the psychological side of visual sensations. If we wish an explanation of them, of their variety and of their peculiarities, we must turn once more to anatomy and physiology.