Page:Astronomy for Everybody.djvu/72

of telescope, one called refracting, the other reflecting. We begin with the former because it is the more usual.

The lenses of a refracting telescope comprise two combinations or systems; the one an object-glass—or "objective," as it is sometimes called for shortness—which forms the image of a distant object in the focus of the instrument; and the other an eyepiece, with which this image is viewed.

The objective is the really difficult and delicate part of the instrument. Its construction involves more refined skill than that of all the other parts together. How great is the natural aptitude required may be judged from the fact that a generation ago there was but one man in the world in whose ability to make a perfect object-glass of the largest size astronomers everywhere would have felt confidence. This man was Alvan Clark, of whom we shall soon speak.

The object-glass, as commonly made, consists of two large lenses. The power of the telescope depends altogether on the diameter of these lenses, which is called the aperture of the telescope. The aperture may vary from three or four inches, in the little telescope which one has in his house, to more than three feet in the great telescope of the Yerkes Observatory. One reason why the power of the telescope depends on the diameter of the object-glass is that, in order to see an object magnified a certain number of times, in its natural brightness, we need a quantity of light expressed by the square