Page:The wonders of optics (1869).djvu/166

 spot for objects at different distances: thus, if you are looking at the moon, and suddenly turn the instrument on to a distant nebula, you will find that the eye-piece requires adjusting. In showing ordinary observers an object in the telescope, it is well to insist on their moving the eye-piece backwards and forwards until distinct vision is obtained, for it often happens that people will say they see an object quite distinctly, when it is in reality misty, and will generally refuse to allow the focus to be altered. It is very singular how human vanity or complaisance will step in when some persons are looking through a telescope. They seem to think that there is some disgrace or rudeness involved in their not being able to see what their predecessors at the instrument have seen. Poor John Leech leaves us an amusing instance of this in a comic cut inserted in one of the early numbers of our old friend Punch. A gentleman is endeavouring to show a lady a distant steamboat through a telescope, but she has it accidentally pointed at two swans that are swimming on the margin of the lake below; consequently when he asks her if she sees the steamer, she replies that "she sees it most distinctly, and there are two of them," a pretty good proof that the instrument was not only pointed at the wrong object, but was out of focus as well.

In constructing a telescope similar to the one described above, the object-glass ought to be of considerable diameter and of long focus; the eye-piece, on the contrary, should be comparatively small and of short focus. A little consideration will show the reason of this. An object-glass of long focus will form a large image at the point a b, and the eye-piece of short focus will magnify this image more than another lens of less convexity. It is, however, on the size, length of focus, and perfection of workmanship of the objective that the