Page:A Voyage in Space (1913).djvu/143

Rh as a conjuring trick—one of those dodges that is so easy when once you know and so difficult before-hand. If any of you likes to test his own brains let him try to think for himself how this was done: and to give you a little time to think I will not tell you the answer just yet. We will look at this picture (p. 124) of the ladies who do the calculations at the Mount Wilson Observatory, ready to ascend the 150-foot tower in the bucket which acts as a lift, and then we will leave the tower for a time and go down into the well, where all kinds of apparatus are available which it would take us several courses of lectures to explain fully, but of which we may get some idea without working too hard.

Let us first think of the three essential parts of a telescope—any telescope—we might take the very first that Galileo looked through. There were two lenses joined by a tube. Now these three separate parts of a telescope have each had histories of their own which we have partly reviewed already. The tube went ahead first; you remember how the tube became immensely long, because of colour, without any great alteration in the lenses: then one of the lenses (the object-glass as it is called, because it is turned to the object: the other is called the eye-lens or eyepiece] became a mirror and was made much larger, while the tube shrank back to normal size. We thereupon followed the history of the mirror, seeing how it became larger and larger: