[Illustration: FIG. 70.--The Grinding Tool.]
When the amateur astronomer has completed this part of the task, all the great difficulties about his telescope are conquered. The tube may be made of wood, and, indeed, a square tube will do just as well as a round one. He must also provide for the top of the tube a small mirror, which has to be perfectly flat. The preparation of this requires much care, because it is not so easy as one might suppose to obtain an accurately flat surface. One way of doing this is to get three pieces, and grind each two of them together until every pair will touch all over; then they will certainly all be flat. One more part you want, and that is an eyepiece. This presents no difficulty. A single glass lens can be made to answer and your telescope is complete.
It was in the year 1774 that Herschel first had a view of the heavens through the telescope he had himself constructed. During the early part of his career he does not seem to have made any important discoveries. He was gradually preparing himself for the great achievement by which his name became famous.
It was on the 13th of March, 1781, that the organist of the Octagon Chapel at Bath turned his telescope on the constellation of the Twins, and began to look at one star after another. You must know that a star merely looks like a little point in a telescope; even the greatest instrument will only make a star look brighter, and will never show it with a perceptible disk. In looking over the stars that night, Herschel’s attention was arrested by one object that did look larger when magnified, and therefore was not a star. The only other objects which would behave in this way were the planets, or possibly a comet. Indeed, at first Herschel imagined that what he saw must be a comet. It could hardly have occurred to him that he was to have such good fortune as to discover a new planet. The five great planets had been known from all antiquity. Was it reasonable to suppose that there could be yet another that had never been perceived? Fortunately, there was a test available. A star remains in the same place from night to night and from year to year; while a planet, as we have already had occasion to mention, is a body which is wandering about. The movements of a planet are, however, not at all like those of a comet. To decide on the nature of Herschel’s newly discovered body, it was sufficient to observe the character of its motion. A few nights sufficed to do this. The position of the body was carefully marked relatively to the neighboring stars, and it was soon shown that it was a planet.