THE MICROSCOPE. 



21 



The indices of refraction, already alluded to ( 21), are 

 really the indices of deflection ; they indicate the amount of 

 deflection that a ray of homogeneous light, e.g. pure red, 

 would undergo when transmitted through various sub- 

 stances. The table of indices (21) shows that in flint 

 glass the deflective power is 1*642, while in crown glass it is 

 i '5 3. It would, however, be easy to have a flint prism 

 that would give the same deflection as a crown prism ; the 

 former only requires to be made of a smaller angle than 

 the latter ; thus a prism of flint glass with a refracting 

 angle of 52 would have the same deflective power as a 

 60 prism of crown glass ; a ray of pure red light would be 

 deflected to the same extent by both. But if a ray of 

 white light were transmitted through each prism, the 

 spectrum produced by the flint would be nearly twice as long 

 as that produced by the crown glass ; that is to say, the dis- 

 persive power of flint is nearly double (17 in exact figures) 

 that of crown glass. 



34. The construction of an achromatic lens then is 

 essentially this. If we combine in opposite positions two 

 prisms, one of crown 

 glass of about 60 (c, 

 Fig. 24), the other (/) 

 of flint glass of about 

 30, a ray (r) passing 

 into the prism c is de- 

 flected and dispersed. 

 On passing into/" it is deflected and dispersed in an op- 

 posite direction. But, ow- 

 ing to the smaller angle 

 of /, the outward (upward 

 in the Fig.) deflection of 

 the ray does not overcome 

 its inward deflection in c, 

 therefore, it emerges with 

 a part of the deflection 

 given to it in c still re- 

 tained. But the dispersive power of flint being nearly 

 double that of crown glass, the outward dispersion in / 



FIG. 24. Achromatic prisms. (Lommel.} 



FIG. 25. Achromatic lens. 



