Page:Astronomy for Everybody.djvu/77

glass, essentially of the same kind that the watchmaker uses to examine the works of a watch. The smaller the eyepiece, the more closely the examination can be made, and the greater the magnifying power.

The question is often asked, how great is the magnifying power of some celebrated telescope. The answer is that the magnifying power depends not only on the object-glass but on the eyepiece. The smaller the latter the greater the magnifying power. Astronomical telescopes are supplied with quite a large collection of eyepieces, varying from the lowest to the highest power, according to the needs of the observer.

So far as the geometric principle goes, we can get any magnifying power we please on any telescope, however small. By viewing the image with an ordinary microscope, such as is used by physicians, we might give a little four-inch telescope the magnification of Herschel's great reflectors. But there are many practical difficulties in carrying the magnification of any instrument above a certain point. First there is the want of light in seeing the surface of an object. If we looked at Saturn with a three-inch telescope, using a magnifying power of several hundred times, the planet would seem dim and indistinct. But this is not the only difficulty in using a high magnifying power with a small telescope. The effect of light having a wave length is such that as a general rule we can get no advantage in carrying the magnification above fifty, or one hundred at the