I have often been asked why it is that a telescope enables us to see objects, both faint and small, which our unaided eyes fail to show. Perhaps this will be a good opportunity to say a few words on the subject. I think we can explain the utility of the telescope by examining our own eyes. The eye undergoes a remarkable transformation when its owner passes from darkness into a brilliantly lighted room ([Fig. 64]). Here you see two views of an eye, and you notice the great difference between them. They are not intended to be the eyes of two different people, or the two eyes of the same person; they are merely two conditions of the same eye. They are intended to illustrate two different states of the eye of a collier. The right shows his eye when he is above ground in bright daylight; the left is his eye when he has gone down the coal-pit to his useful work in the dark regions below. I remember when I went down a coal-pit I was lowered down a long shaft, and when the bottom was reached a safety lamp was handed to me. The gloom was such, that at first I found some little difficulty in guiding my steps, but the capable guide beside me said in an encouraging voice, “You will be all right, sir, in a few moments, for you will get your pit-eyes.” I did get my “pit-eyes,” as he promised, and was able to see my way along sufficiently to enjoy the wonderful sights that are met with in the depths below.
This shows the Eye in
the Dark.
This shows the Eye in
the Daylight.
Fig. 64.
The change that came over my eyes is that which these two pictures illustrate: the black, round spot in the centre is an opening covered with a transparent window, by which light enters the eye; the black spot is called the pupil, and nature has provided a beautiful contrivance by which the pupil can get larger or smaller, so as to make vision agreeable. When there is a great deal of light we limit the amount that enters by contracting the pupil so as to make the opening smaller. Thus the picture with the small pupil represents the state of the collier’s eye when he is above ground in bright sunlight. When he descends into the pit, where the light is very scanty, then he wants to grasp as much of it as ever he can, and consequently his pupil enlarges so as to make a wider opening, and this is what he calls getting his “pit-eyes.”
But you need not go down a coal-mine to see the use of the iris—for so that pretty membrane is called which surrounds the pupil. Every time you pass from light into darkness the same thing can be perceived. When we turn down the lights in a room, so that we are in comparative darkness, our pupils gradually expand. As soon as the lights are turned up again, then our pupils begin to contract. Other animals have the same contrivance in their eyes. You may notice in the Zoölogical Gardens how quickly the pupil of the lion contracts when he raises his eyes to the light. The power of rapidly changing the pupil might be of service to a beast of prey. Imagine him crouching in a dense shade to wait for his dinner; then of course the pupil will be large from deficiency of light; but when he springs out suddenly on his victim, in bright light, it would surely be of advantage to him to be able at once to see clearly. Accordingly his pupil adjusts itself to the altered conditions with a rapidity that might not be necessary for creatures of less predaceous habits.
These changes of the pupil explain how the telescope aids our eyes when we want to discern any faint objects, like the little planets. Such bodies are not visible to the unaided eyes, because our pupils are not large enough to grasp sufficient light for the purpose. Even when they are opened to the utmost, we want something that shall enable them to open wider still. We must therefore borrow assistance from some device which shall have an effect equivalent to an enlargement of the pupil beyond the limits that nature has actually assigned to it. What we want is something like a funnel which shall transform a large beam of rays into a small one. I may explain what I mean by the following illustration: Suppose that it is raining heavily, and that you want to fill a bucket with water. If you merely put the bucket out in the middle of a field, it will never be filled; but bring it to where the rain-shoot from a house-top is running down, and then your bucket will be running over in a few moments. The reason, of course, is that the broad top of the house has caught a vast number of drops and brought them together in the narrow shoot, and so the bucket is filled. In the same way the telescope gathers the rays of light that fall on the object glass, and condenses them into a small beam which can enter the eye. We thus have what is nearly equivalent to an eye with a pupil as big as the object glass. Thus the effect of a grand telescope amounts to a practical increase of the pupil from the size of a threepenny-piece up to that of a dinner-plate, or even much larger still.
THE ASTEROIDS OR SMALL PLANETS.
An asteroid is like a tiny star, and in fact the two bodies are very often mistaken. If we could get close to the objects, we should see a wide difference between them. We should find the asteroid to be a dark planet like our earth, lighted only by the rays from the sun. The star, small and faint though it may seem, is itself a bright sun, at such a vast distance that it is only visible as a small point. The star is millions of times as far from us as the planet, and utterly different in every respect.