28 CONSTRUCTION OP THE MICROSCOPE. 



respectively refract and disperse the rays of light ; so that 

 the one medium may, by equal and contrary dispersion, 

 neutralize the dispersion caused by the other, without, at 

 the same time, wholly neutralizing its refraction. Remark- 

 able enough, the media found the most valuable for such 

 a purpose should be the combination of pieces of crown 

 and flint glass, of crown-glass whose index of refraction is 

 1-519, and dispersive power 0-036, and of flint-glass whose 

 index of refraction is 1-589, and dispersive power 0-0393. 

 The focal length of the convex crown-glass lens must be 

 4^ inches, and that of the concave flint-glass lens 7f 

 inches, the combined focal length of which is 10 inches. 

 The following fig. 17 will serve us to explain how a ray 

 of light is brought to a single focus, free from colour. 



Fig. 17. 



In this diagram, L L is a convex lens of crown-glass, and 

 I I a concave one of flint-glass. A ray of light (s) falling at 

 F on a convex lens, will refract it exactly in the same 

 manner as the prism ABC, whose faces touch the two 

 surfaces of the lens at the points where the ray enters, and 

 quits. The ray s F, thus refracted by the lens L L, or 

 prism ABC, would have formed a spectrum (P T) on a 

 screen or wall, had there been no other lens, the violet ray 

 (F v) crossing the axis of the lens at v, and going to the 

 upper end (P) of the spectrum ; and the red ray (F R) 

 going to the lower end (T). But, as the flint-glass lens (I I) 

 or the prism A a c, which receives the rays F v, F R, at the 

 same points, is interposed, these rays will be united at /, 

 and form a small circle of white light, the ray (s F) being 

 now refracted without colour from its primitive direction 



