Page:A Voyage in Space (1913).djvu/145

Rh the most eventful of the three, because there has been much more variety in it. The object end and the tube have changed chiefly in size; but the eye end had become a photographic plate, or a spectroscope, or a photometer, or a spectroheliograph, or any one of a number of such things which can be attached at the eye end. One of the practical problems of an astronomer is to change one of these pieces of apparatus for another as easily and rapidly as possible: and Professor Hale has devised the best method up to the present. Perhaps some of you have on your breakfast-tables a dumb-waiter which can be turned round, so that you can get at the butter or the toast or the marmalade when you want it? There is a contrivance of a similar kind down in the 80-foot well below the 150-foot tower. Of course it is much bigger and stronger than your breakfast-table apparatus: but in just the same way, and almost as easily, as you help yourself to toast first and then jam, Professor Hale can put on the spectroheliogaph first and then the polarimeter, say, or whatever he wants to use.

Up to the present, the only one of these things that we have said much about is the photographic plate. We can take away the eyepiece of a telescope (and also the eye that looks through it) and substitute a photographic plate, and some of the advantages of doing this have already been mentioned, especially that the plate can look at much more of the sky at one time. We also mentioned that the plate could be exposed for many hours, and in this it has another advantage over the eye, because it is steadily photographing more and more (that is,