During the last century telescope construction underwent an unprecedented development. An immense amount of interest was taken in the construction of large telescopes, and the different countries of the world entered on an exciting race to produce the most powerful possible instruments. Besides this rivalry of different countries there was a rivalry of methods. The telescope developed along two different lines, and each of these two types has its partisans at the present day. These types are known as _refractors_ and _reflectors_, and it is necessary to mention, briefly, the principles employed in each. The _refractor_ is the ordinary, familiar type of telescope. It consists, essentially, of a large lens at one end of a tube, and a small lens, called the eye-piece, at the other. The function of the large lens is to act as a sort of gigantic eye. It collects a large amount of light, an amount proportional to its size, and brings this light to a focus within the tube of the telescope. It thus produces a small but bright image, and the eye-piece magnifies this image. In the _reflector_, instead of a large lens at the top of the tube, a large mirror is placed at the bottom. This mirror is so shaped as to reflect the light that falls on it to a focus, whence the light is again led to an eye-piece. Thus the refractor and the reflector differ chiefly in their manner of gathering light. The powerfulness of the telescope depends on the size of the light-gatherer. A telescope with a lens four inches in diameter is four times as powerful as the one with a lens two inches in diameter, for the amount of light gathered obviously depends on the _area_ of the lens, and the area varies as the _square_ of the diameter. The largest telescopes at present in existence are _reflectors_. It is much easier to construct a very large mirror than to construct a very large lens; it is also cheaper. A mirror is more likely to get out of order than is a lens, however, and any irregularity in the shape of a mirror produces a greater distorting effect than in a lens. A refractor is also more convenient to handle than is a reflector. For these reasons great refractors are still made, but the largest of them, the great Yerkes' refractor, is much smaller than the greatest reflector, the one on Mount Wilson, California. The lens of the Yerkes' refractor measures three feet four inches in diameter, whereas the Mount Wilson reflector has a diameter of no less than eight feet four inches. [Illustration: THE YERKES 40-INCH REFRACTOR (The largest _refracting_ telescope in the world. Its big lens weighs 1,000 pounds, and its mammoth tube, which is 62 feet long, weighs about 12,000 pounds. The parts to be moved weigh approximately 22 tons. The great _100-inch reflector_ of the Mount Wilson reflecting telescope--the largest _reflecting_ instrument in the world--weighs nearly 9,000 pounds and the moving parts of the telescope weigh about 100 tons.