194 Experimental Philosophy. [Lecture 13. 



distance of the eye-glass, and the quotient ex- 

 presses the magnifying power. 



The greatest inconvenience attending dioptric 

 or refracting telescopes was found to be that 

 which arises from what is called the aberration 

 of light, which, when high magnifiers were used, 

 that is, lenses much thicker in the middle than 

 at the sides, produced often a confused, and 

 sometimes a coloured image. This effect is the 

 result of refraction, and it consists in different 

 rays, according to their obliquity, uniting in dif- 

 ferent foci, though proceeding through the same 

 lens. This will be easily understood by fig. 74. 

 Suppose, then, PP to be a convex lens, and E e 

 an object, the point E of which corresponds with 

 the axis of the lens, and sends forth the rays 

 EM, EN, EA, EM, EN, all of which reach the 

 surface of the glass, but in different parts. The 

 ray EA, which penetrates the centre of the glass, 

 suffers no refraction ; the rays EM, EM, which 

 pass near EA, will be converged to a focus at F 

 But the rays EN, EN, which strike more ob- 

 liquely near the edges of the glass, will be differ- 

 ently refracted, and will meet about G, nearer to 

 the lens, where they will form another image Gg. 

 In this manner several images will be formed in 

 different foci ; and though to the eye which looks 

 through the lens one image only will be apparent, 

 yet that image, from being composed of so many 

 combined, will be confused and distorted. 



