48 The Outline of Science 



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 pf 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 power- 

 fulness 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 con- 

 struct 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 irregu- 

 larity 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. 



