But how this image or picture gets to the mind is another question; a question, I am sorry to say, which nobody can altogether answer. We do know, however, that there are nerve endings in the retina, something like hot spots, cold spots, touch spots, and pain spots in the skin, only of course very much nearer together. Probably there are three kinds of these—red spots, green spots, and blue spots. Each spot sees one color; and by combining these colors in all sorts of ways, we build up the complicated pictures which we see. Still it is by no means impossible that there may be, not three, but six elements in our eye-pictures—white, black, red, yellow, green, and blue. Nobody really knows; and it all shows how little, after all, we have succeeded in finding out about ourselves, in spite of whole lifetimes of study of many hundreds of scientific men. Who knows but that some of you who read these pages may be the ones to discover some of these things which all the world thus far has not been able to learn.
There are still other curious facts about our sight which anybody can make out for himself. If you take any colored object, this book for example, put it behind your head; and then slowly bring it round in front, while you keep your eyes looking out steadily straight forward, you will notice certain very peculiar facts. In the first place, you will discover that you can see surprisingly far toward the back of your head. A horse can see all the way round, and if he did not wear blinders, could watch the people in the carriage behind and the road which stretches out in front, along with everything in between, all at once and about equally well. Many animals, in short, can see clear round their heads. We can’t; we can see only about half way round.
Then you will notice that you can see that something is there and moving, while it is still so far round to the side that you cannot at all make out either its shape or its color. Furthermore, you can see the color perfectly well, long before you can make out the exact shape. Indeed, you can make out the shape of ordinary letters well enough to read them only when you hold them exactly in front of the eye. The least little movement out of that one small spot mixes a whole page to a gray blur; curiously too, you can make out blue and green decidedly farther round toward the corner of the eye than you can tell red.
In short, then, we can see movement considerably farther round toward the backs of our heads than we can see color. We can see blue farther round than we can see green and green farther round than we can see red. But we cannot see shapes accurately, except right in front of our noses.
Now curiously enough, all animals that can see at all, can see something moving; tho they cannot see colors at all perfectly, nor make out the shape of anything. Many lowly sea creatures have eyes of this sort. A better kind of eye, like those of many insects, can see colors, but not make out much about shapes; while certain ants can see blue and green but are blind to red. Few indeed are the creatures that can see anything like as clearly as we see, looking hard at an object straight in front. Even a dog cannot do it, nor a horse.
XXVII
Seeing and Believing
Even we ourselves, we human beings, by no means always see so truly as we think we do. Take a look at this figure and say which of the two lines in the south-west corner continues the single line in the north-east corner. Then lay on a ruler or a strip of paper, and see which line really does run clear across the figure.
Or look at the figure below, and say which way the curved lines bend. Then take a straight-edge and test them.