96 TKAIfSACTIONS OF THE [FEB. 21, 



make our magnifying power as great as we choose. If, in the 

 case cited, we use an eye-lens with a focal length of say xot^^^ ^^ 

 an inch, we obtain a magnifying power of 100 X 100, or 10,000 

 diameters. " But in attempting to do this a difficulty arises 

 with which astronomers have always had to contend, and which 

 has its origin in the imperfection of the image formed by the 

 object-glass. No lens will bring all the rays of light to abso- 

 lutely the same focus. When light passes through a prism the 

 ■various colors are refracted unequally, red being refracted the 

 least, and violet the most. 



*'It is the same when light is refracted by a lens, and the 

 consequence is that the red rays will be brought to the farthest 

 focus and the violet rays to the nearest, while the intermediate 

 colors will be scattered between. As the light is all not brought to 

 the same focus, it is impossible to get any accurate image of a 

 star or other object, at which the telescope is pointed. The eye 

 sees only a confused mixture of images of various colors. When 

 a sufficiently low magnifying power is used, the confusion will 

 be slight, the edges of the object being indistinct and made up 

 of colored fringes. When the magnifying power is increased, the 

 object will indeed look larger, but these confused fringes will 

 look larger in the same proportion, so that the observer will see 

 no more than before. This separation of light in a telescope is 

 called chromatic aberration." 



The early astronomers found no way to get rid of this diffi- 

 culty. They discovered, however, that they could diminish the 

 trouble by increasing the focal length of the telescope, and thus 

 making the image larger. An object-glass, say, of 5 inches 

 diameter, with focal length of 60 feet, would give no more con- 

 fused image than the same object-glass with a focal length of 6 

 feet. The image formed by the first would be ten times as large 

 as that formed by the second, so that a low power of eye-lens 

 could be used, and hence the confused fringes produced the less 

 disturbing effect with a given eye-lens, the greater the focal 

 length of the object-glass. In this way Hughens, Cassini, 

 Hevelius, Blanchini and other astronomers of the 17th century 

 were able to obtain quite high magnifying powers. These 

 astronomers made telescopes of 100 to 150 feet in focal length. 



