154 Mr Pritchard on forming Diamond Lenses 



substance, and its expence, which is greatly enlarged by the 

 previous process of determining its goodness for optical purpos- 

 es, or if we risk the stone (as I have often been tempted to do) 

 its entire loss when the lens is completed, should it happen to 

 possess polarity in the direction of its axis, which render the 

 discovery of a substitute highly important. 



It has long ago been determined by Dr Brewster, in his 

 "Work on new philosophical instruments, that the precious stones 

 offer the best known materials for the formation of magnifiers 

 on account of their feeble dispersion, combined with the high 

 refractive indices of those substances. It will at first appear 

 easy to select that stone which has the strongest refractive 

 power ; but here we have to contend with another property 

 which can be avoided in the diamond, viz. the colour of the 

 stone, and although this will be very little in deep magnifiers, 

 yet it then becomes more necessary to avoid those stones that 

 do not transmit the rays found by experience most essential for 

 examining the intimate structure of very delicate and minute 

 bodies. Now, the substance of which I form my lenses can be 

 selected nearly free from any colour, while the rays which re- 

 main appear, from the experiments of Dr Wollaston, to be ad- 

 mirably suited for viewing the most minute if not the ultimate 

 organization of animal and vegetable tissues ; for, in the beauti- 

 ful and effective method of illumination adopted by him, of se- 

 parating by means of a convex lens the white light, and ad- 

 justing the focus in such a manner that the object shall only be 

 illuminated by the violet rays, he was enabled to command 

 at pleasure the vision of the most delicate markings of different 

 test objects, a thing extremely difficult even with the best mi- 

 croscopes and the ordinary illumination. A short time before 

 his decease he showed me several objects, both with glass doub- 

 lets and my sapphire lenses, illuminated by his method, which 

 certainly exhibited them in a very satisfactory manner, and with- 

 out any uncommon management, but which objects require 

 great care for their developemcnt in the ordinary mode of view- 

 ing them. The sapphires I employ are almost colourless, re- 

 taining only a tinge of violet, which greatly adds to their value, 

 as the complementary colour would diminish, while at the same 

 time it is less fatiguing to the eye than looking through the lat- 



