Page:Astronomy for Everybody.djvu/91

Although the refracting telescope is that in most general use, there is another form of instrument of radically different construction. Its main feature is that the functions of the object-glass are performed by a slightly concave mirror. That such a mirror reflects parallel rays falling upon it to a focus, is doubtless well known to our readers. The focus is situated about half-way between the mirror and its centre of curvature.

This form of instrument has an enormous advantage in its freedom from the "secondary aberration" which we have already described as inherent in the refracting telescope. Another advantage which it possesses is that it can be made of larger dimensions than the other. The extreme limit so far reached in the refractor, as we have already stated, is four feet; but the forty-inch aperture of the Yerkes telescope is, up to the present time, the limit in actual use for astronomical research. But, more than half a century ago. Lord Rosse constructed his great reflector of six feet diameter. Judging by its size alone, this instrument ought to give several times more light, and therefore show far minuter stars, than any refracting telescope yet made. But, for some reason, its performance—and, indeed, that of reflectors generally—has not corresponded to the size.