TRANSACTIONS OF THE SECTIONS. .177 



Srdly, That they are as nearly as possible in the same ratio. 



In theory, the principles on which achromatism might be 

 produced in a combination similar to the eye are easily under- 

 stood by reference to the common formula for the dispersion 

 of a single lens. Let m, mr, m„, Sec, be the absolute indices, 

 relative to vacuum, of the medium of the lens, for the mean, 

 red, violet, &c., rays. 



The common formula applies to the focus for parallel rays 

 formed in vacuo. Let us now suppose it formed in some other 

 medium, whose absolute indices (as before) are m^ mn, m„j, &c., 

 and /the principal focal length of the lens for mean rays in this 

 medium. Then, instead of the common formula for the chro- 

 matic aberration («), we shall have the analogous one 



" m„ mr "I 



Here the numerator may be = 0, or the aberration be de- 

 stroyed, whatever be the absolute values of the indices, pro- 

 vided only the indices of corresponding rays are in the same 

 ratio in the two media. 



By a construction of the course of the rays it will be easily 

 seen, that with certain dispersive powers and curvature the 

 rays will reunite at given distances. 



This appears to be the same principle as that on which Sir 

 David Brewster proposed an achromatic microscope. {Phil. 

 Instruments, p. 408.) 



.On the Power of Glass of Antimony to reflect Light. By 



R. Potter, Jun. 



In an abstract of a paper which is published in the preceding 

 volume of these Reports, Mr. Potter gave the results he had 

 obtained of the reflective power of diamond for light, by the 

 method of photometry by comparison. Since then he had 

 experimented with a large larke diamond belonging to Sir 

 David Brewster, and obtained results nearly identical. He 

 found this latter diamond to reflect at 2° to 3° incidence 

 equally with crown-glass at 64° incidence, from which the va- 

 rious trials differed very little. 



The ordinary surfaces on diamond being too small and the 

 planes not sufficiently true to determine the reflective power at 

 high incidences, he had recourse to glass of antimony, a sub- 

 stance of very high refractive power, to which he succeeded 



