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do not trouble us on the small scale, they become obtrusive in the enlargement. In the same way if you try to enlarge the image made in a telescope beyond a certain point the defects become glaring and you get no advantage. These defects are due partly to fault in the making of the lens or mirror (because no workmanship can be quite perfect) and partly to the tremulousness of our atmosphere. All kinds of cross currents are continually passing in the air through which we are compelled to look at the heavens; and they all tend to blur and confuse the image. When we do not magnify it much we do not see these blurs; but a high magnification makes them obvious.

This illustration of the jockey reminds us of another fact about a large telescope: we cannot see much at once. In the original picture we see not only the jockey but the horse as well, but when we enlarge it, we must be content with the jockey's head if we are to have the patch of the same size. Perhaps you will ask why we need have this limited patch at all? Why not make the whole picture bigger? I am trying to show you what you actually see when you look through a big telescope: and you will always find your vision bounded by a ring, which limits the "field of view" as it is called. In using a telescope you have to put your eye to a small hole in the eyepiece. Fig. 31 is a picture of the great nebula in Andromeda, and the little hole in the covering screen enables us to see just about as much of it as we should see at one time in a large telescope. If we move the telescope about, we can change