"But by going to the proper end of the telescope you can get quite near the image, and can see and magnify it, if you put a strong lens to collect the rays from it to a focus. This is the use of the eye-piece, which in our diagram is placed at a quarter of a foot or three inches from the image in both telescopes. Now that we are close to the images, the divergence of the points i, i makes a great difference. In the small telescope, in which the image is only one foot behind the object-glass, the eye-piece being a quarter of a foot from it, is four times nearer, so the angle i, o, i is four times the angle i, x, i, and the man looking through it sees the image magnified four times. But in the longer telescope the image is two feet behind the lens, while the eye-piece is, as before, a quarter of a foot from it. Thus the eyepiece is now eight times nearer, so the angle i, o, i is eight times the angle i, x, i, and the observer sees the image magnified eight times.
"In real telescopes, where the difference between the focal length of the object-glass and that of the eye-glass can be made enormously greater, the magnifying power is quite startling, only the object-glass must be large, so as to collect enough rays to bear spreading widely. Even in your small telescopes, with a focus of eighteen inches, and an object-glass measuring one and a quarter inch across, we can put on a quarter of an inch eye-piece, and so magnify seventy-two times; while in my observatory telescope, eight feet or ninety-six inches long, an eye-piece of half an inch magnifies 192 times, and I can put on a 1/8-inch eye-piece and magnify 768 times! And so we can go on lengthening the focus of the object-glass and shortening the focus of the eye-piece, till in Lord Rosse's gigantic fifty-six-foot telescope, in which the image is fifty-four feet (648 inches) behind the object-glass, an eye-piece one-eighth of an inch from the image magnifies 5184 times! These giant telescopes, however, require an enormous object-glass or mirror, for the points of light are so spread out in making the large image that it is very faint unless an enormous number of rays are collected. Lord Rosse's telescope has a reflecting mirror measuring six feet across, and a man can walk upright in the telescope tube. The most powerful telescope yet made is that at the Lick Observatory, on Mount Hamilton, in California. It is fifty-six and a half feet long, the object-lens measures thirty-six inches across. A star seen through this telescope appears 2000 times as bright as when seen with the naked eye.
"You need not, however, wait for an opportunity to look through giant telescopes, for my small student's telescope, only four feet long, which we carry out on to the lawn, will show you endless unseen wonders; while your hand telescopes, and even a common opera-glass, will show many features on the face of the moon, and enable you to see the crescent of Venus, Jupiter's moons, and Saturn's rings, besides hundreds of stars unseen by the naked eye.
"Of course you will understand that Fig. 18 only shows the principle of the telescope. In all good instruments the lenses and other parts are more complicated; and in a terrestrial telescope, for looking at objects on the earth, another lens has to be put in to turn them right way up again. In looking at the sky it does not matter which way up we see a planet or a star, so the second glass is not needed, and we lose light by using it.
"We have now three magic glasses to work for us—the magnifying-glass, the microscope, and the telescope. Besides these, however, we have two other helpers, if possible even more wonderful. These are the Photographic camera and the Spectroscope."
Fig. 19.
Photographic camera.
l, l, Lenses. s, s, Screen cutting off diverging rays. c, c, Sliding box. p, p Picture formed.
"Now that we thoroughly understand the use of lenses, I need scarcely explain this photographic camera (Fig. 19), for it is clearly an artificial eye. In place of the crystalline lens (compare with Fig. 11) the photographer uses one, or generally two lenses l, l, with a black ledge or stop s between them, which acts like the iris in cutting off the rays too near the edge of the lens. The dark camera c answers to the dark chamber of the eyeball, and the plate p, p at the back of the chamber, which is made sensitive by chemicals, answers our retina. The box is formed of two parts, sliding one within the other at c, so as to place the plate at a proper distance from the lens, and then a screw adjusts the focus more exactly by bringing the front lens back or forward, instead of altering the curve as the ciliary muscle does in our eye. The difference between the two instruments is that in our eye the message goes to the brain, and the image disappears when we turn our eyes away from the object; but in the camera the waves of light work upon the chemicals, and the image can be fixed and remain for ever.