Through Magic Glasses and Other Lectures / A Sequel to The Fairyland of Science

by Arabella B. Buckley

ep, Eye-piece. og, Object-glass. f, Finder.

"A Telescope (Fig. 17) can, like the microscope, be made of only two glasses: an object-glass to form an image in the tube and a magnifying eye-piece to enlarge it. But there is this difference, that the object lens of a microscope is put close down to a minute object, so that the rays fall upon it at a wide angle, and the image formed in the tube is very much larger than the object outside. In the telescope, on the contrary, the thing we look at is far off, so that the rays fall on the object-glass at such a very narrow angle as to be practically parallel, and the image in the tube is of course very, very much smaller than the house, or church, or planet it pictures. What the object-glass of the telescope does for us, is to bring a small real image of an object very far off close to us in the tube of the telescope so that we can examine it.

"Think for a moment what this means. Imagine that star we spoke of (p. 41), whose light, travelling 186,300 miles in one second, still takes 2000 years to reach us. Picture the tiny waves of light crossing the countless billions of miles of space during those two thousand years, and reaching us so widely spread out that the few faint rays which strike our eye are quite useless, and for us that star has no existence; we cannot see it. Then go and ask the giant telescope, by turning the object-glass in the direction where that star lies in infinite space. The widespread rays are collected and come to a minute bright image in the dark tube. You put the eye-piece to this image, and there, under your eye, is a shining point: this is the image of the star, which otherwise would be lost to you in the mighty distance.

"Can any magic tale be more marvellous, or any thought grander, or more sublime than this? From my little chamber, by making use of the laws of light, which are the same wherever we turn, we can penetrate into depths so vast that we are not able even to measure them, and bring back unseen stars to tell us the secrets of the mighty universe. As far as the stars are concerned, whether we see them or not depends entirely upon the number of rays collected by the object-glass; for at such enormous distances the rays have no angle that we can measure, and magnify as you will, the brightest star only remains a point of light. It is in order to collect enough rays that astronomers have tried to have larger and larger object-glasses; so that while a small good hand telescope, such as you use, may have an object-glass measuring only an inch and a quarter across, some of the giant telescopes have lenses of two and a half feet, or thirty inches, diameter. These enormous lenses are very difficult to make and manage, and have many faults, therefore astronomical telescopes are often made with curved mirrors to reflect the rays, and bring them to a focus instead of refracting them as curved lenses do.

"We see, then, that one very important use of the telescope is to bring objects into view which otherwise we would never see; for, as I have already said, though we bring the stars into sight, we cannot magnify them. But whenever an object is near enough for the rays to fall even at a very small perceptible angle on the object-glass, then we can magnify them; and the longer the telescope, and the stronger the eye-piece, the more the object is magnified.

"I want you to understand the meaning of this, for it is really very simple, only it requires a little thought. Here are skeleton drawings of two telescopes (Fig. 18), one double the length of the other. Let us suppose that two people are using them to look at an arrow on a weathercock a long distance off. The rays of light r, r from the two ends of the arrow will enter both telescopes at the same angle r, x, r, cross in the lens, and pass on at exactly the same angle into the tubes. So far all is alike, but now comes the difference. In the short telescope A the object-glass must be of such a curve as to bring the cones of light in each ray to a focus at a distance of one foot behind it, [1] and there a small image i, i of the arrow is formed. But B being twice the length, allows the lens to be less curved, and the image to be formed two feet behind the object-glass; and as the rays r, r have been diverging ever since they crossed at x, the real image of the arrow formed at i, i is twice the size of the same image in A. Nevertheless, if you could put a piece of paper at i, i in both telescopes, and look through the object-glass (which you cannot actually do, because your head would block out the rays), the arrow would appear the same size in both telescopes, because one would be twice as far off from you as the other, and the angle i, x, i is the same in both."

Fig. 18.

Skeletons of telescopes.

A, A one-foot telescope with a three-inch eye-piece.
B, A two-foot telescope with a three-inch eye-piece.
e, p, Eye-piece. o, g, Object-glass. r, r, Rays which enter the telescopes and crossing at x form an image at i, i, which is magnified by the lens e, p. The angles r, x, r and i, x, i are the same. In A the angle i, o, i is four times greater than that of i, x, i. In B it is eight times greater.