ceeded in making lenses larger and larger until the largest is now 40 inches wide—the great lens at the Yerkes Observatory near Chicago. That is a long way short of the largest mirror telescope (you remember that Lord Rosse's was six feet wide); but it is far easier to make a mirror than a lens. You see the mirror has only one surface to be made; you must be careful to get that surface as nearly perfect as possible, but when that is done your work is over. But a lens has at least four surfaces; for to get rid of colour we must put two lenses together, balancing their colour effects, but leaving the focussing effects unbalanced. Each one of these four surfaces must be as carefully made as the single surface of the mirror; and besides, we have to be very careful that the glass inside is as perfect as possible, since a slight flaw in it will make trouble and confusion at the focus of the telescope. You may ask why, then, do astronomers bother to make lenses at all, since mirrors are so much easier? Well, there are several reasons, but I will mention only one: a lens can be made to show more of the sky at once. When we look through a mirror-telescope (or, let us say, when we photograph with it, for that makes the explanation simpler), then we see that the objects in the middle of the picture are in good focus, but not those at the edges. Perhaps many of you have got cameras of your own, kodaks or brownies, or some other sort, and you may know the difference between good and bad lenses—with a good lens all the picture is in focus, with a poor one the edges show up fuzzy. Well, with a mirror we cannot help the edges being fuzzy, because we have nothing we can alter. If we try to alter the
