Why Can’t I See in the Dark?
“The dark” is the absence of light. Now, what is the name for the absence of sound? What do we call the state of things when we hear no sound? The answer is silence. By darkness we mean absence of light, just as silence is absence of sound.
But there is more to say. There may be a wave motion in the ether, but it is hardly proper to call that light until someone sees it. Similarly, there may be a wave movement in the air, but it is hardly proper to call that sound unless someone actually hears it. Seeing and hearing, then, depend, ‘first of all, on there being something outside of us - a particular kind of wave; and secondly, on our being able to feel that something.
A blind man cannot see, even in the light. Our great poet Milton, in his poem on Samson, makes Samson say, when he had lost his sight: “Oh, dark, dark, dark amid the blaze of noon”. That famous line will help us to understand what darkness may depend on - either the absence of light or the absence of the power to see light.
It is often supposed that cats and tigers can “see in the dark,” but we must know that nobody at all can see if it is perfectly dark - that is to say, if there is no light at all. When we speak of being in the dark we usually mean that there is so little light at we see hardly anything.
That is because our eyes are so made that they cannot alter themselves to suit the conditions of very dim light; but some animals can make the pupil of the eye so wide as to get the benefit of whatever rays of light are about. This is the case with cats, and if we watch the cat’s eye when it is in the dark, we see that the pupil appears much enlarged. This allows all the light possible to enter the eye, and the cat, and lions and tigers and other night-prowling animals that have eyes like the cat, are able to see very much better in dim light than we can. But even among human beings there are some people, especially seamen, who can see farther in the dark than others.
It has been discovered that there is light which we cannot see. Our eyes are keyed to a certain scale or band of wave lengths. Beyond this band are shorter waves, which we call ultra-violet rays, still shorter ones, which we call X-rays, and unbelievably tiny ones, gamma rays. There may even be shorter ones than the gamma rays, called cosmic rays. Beyond the other end of the scale of light rays that are visible to us are invisible rays with longer waves. They are called infra-red rays. Instruments can detect these very short and very long rays, though our eyes cannot. Some insects are thought to see them. Some cameras can take pictures by infra-red light, which are the very long light rays; and you probably have seen X-ray photographs, which are pictures taken by extremely short waves.
