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George Biddell Airy
Published by Good Press, 2021
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may serve to illustrate the construction of the telescope. There is no tube, but that is immaterial. At A is what we call a lens, that is to say, a piece of glass convex on both sides, and therefore thickest in the middle. It is here supposed to be fixed in a hole in a wooden screen MN. The property of this lens of glass is, if there be a luminous object in the distance, it collects all the light from that object; and instead of suffering it to go out in a broad sheet of light, it makes it contract so that the light from each point in the object is collected at a corresponding point on the screen; and therefore all the corresponding points of light on the screen, which belong to the original points of light in the original luminous appearance, when put together, form an image which is exactly similar to the original object. The image, however, is turned upside down, because the light which comes from the upper part of the luminous object and goes through the lens, passes downwards towards the lower part of the screen KL. These properties of a lens can easily be proved by experiments with a common burning glass, or a reading glass, or spectacle glass, such as is used by elderly people.
Suppose, now, that the stand GH is placed on the south side of A, and that a lamp is slid along it successively from B to C, D, E, and F. This movement exactly imitates the apparent movement of the stars as they pass across the south, travelling from the east to the west. The effect of it is, that if the screens are placed at proper distances, a spot of light is seen on the screen KL, moving in the opposite direction, as from b, successively to c, d, e, and f. Now, if there are traced upon KL a set of bars or dark wires, the spot of light passes over them in succession, first over one and then over another. Now this is truly and veritably an astronomical telescope. At A is the lens forming the image of the star on KL is the set of wires in the field of view if you placed an eye-glass on the other side of KL, and viewed the wires with it, you would have a complete astronomical telescope. This is the arrangement by which astronomical observations are really and truly made. Every astronomical telescope intended for accurate observations is fitted up with wires of this kind.
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