of Sapphire and Diamond. 329 



paratively shallow curves ; the indistinctness occasioned by the 

 figure of the lens is thus greatly diminished, and the disper- 

 sion of colour in the substance being as low as that of water, 

 renders the lens nearly achromatic." 



The advantages arising from diamond, sapphire, and ruby 

 lenses, will be at once seen from the following measures of 

 their refractive powers, as established by Dr Brewster : — 



Index of refraction. Dispersive power. 



Diamond, 2.470 0.38 



Sapphire, 1.780 0.26 



Ruby, 1.779 0.26 



Plate glass, 1.525 0.32 



In this table the superiority of the diamond is very obvious ; 

 while it produces, in virtue of its low dispersive power, very 

 little colour, its enormous refractive index enables the artist 

 to produce a high magnifying power with very shallow curves. 



The sapphire and the ruby, though they have not the same 

 advantage as the diamond in giving the same magnifying 

 power with as shallow curves, yet they have another valuable 

 property in greater perfection than the diamond, namely, a 

 very low dispersive power. 



On the other hand, the diamond has again the superiority 

 over the sapphire and ruby lenses by its generally having no 

 double refraction, whereas the former have a considerable 

 double refraction ; and we presume Mr Pritchard found it ab- 

 solutely necessary to make the axis of his sapphire lenses co- 

 incident with their axis of double refraction, which is parallel 

 to the axis of the acute rhomboid in which these gems crystal- 

 lize. But even if this is effected, the transmitted rays cannot 

 all pass through the lens parallel to its axis, so that they must 

 to a certain minute degree be separated into two pencils ; but 

 to what extent this will affect the performance of the lens as a 

 microscope we do not yet know. 



But though the diamond may be said to have no double re- 

 fraction when perfectly crystaUized, yet, in nine cases out of 

 ten, Dr Brewster has discovered in it a doubly refracting 

 structure. (See Edinburgh Transactions, vol. viii. p. 157, and 

 Edinburgh Philosophical Journal, vol. iii. p. 98.) Mr Pritchard 



