On this principle is constructed one of the instruments that replace the eye end of the telescope. It has a terribly long name—the spectro-heliograph: spectro from the colours, helio from the Sun, which is usually the picture experimented upon; graph because the records are written by photography. The thing it does is to make a picture of the Sun in one colour, just as we used only yellow light to look at this picture. Then all the parts which have any yellow (or red or whatever colour we choose) show up, while the rest remain black. We can tell at a glance whereabouts there is any sodium (or hydrogen or whatever gives that colour) in the Sun. Suppose we could apply this process to the Earth and take a picture of it showing just where any gold was, would that be a good plan, do you think? London would be a bright spot, and New York and other big cities; places like Alaska would show up, if the gold is not yet all dug out; and then there would be other spots which nobody knows of yet, where the gold is still in the ground,—what a rush there would be to them! The pictures of the Sun do not show us the distribution of anything so exciting as gold, but what they show us is nevertheless of great interest to astronomers, and we will say more of it when we visit the Sun.
I will tell you about just one more of the things we can put on the end of a telescope—something rather simple? this time because perhaps you may think we are getting too complicated for comfort. The point of this instrument is merely its great sensitiveness—it detects variations in light which the eye cannot notice. There is a star called Algol
