Dr, WollastorCs Microscopic Doublet. 259 



from colour, and uniting an enormous refractive power with a 

 low dispersive one. The garnet (was it not for its deep red 

 colour, even when worked into very small lenses) would also 

 be a material admirably calculated for making lenses, for it has 

 not a trace of double refraction about it, and its refractive 

 power is intermediate between sapphire and diamond, while 

 its dispersion is little superior to that of glass : the first lenses 

 formed of precious stones were made many years ago of this 

 substance by Dr. Brewster, and he considers them superior to 

 sapphires. I have not seen any myself, but, doubtless, they 

 would be so, did not their darkness disqualify them for every 

 practical purpose, save that of making lenses for viewing the 

 sun : two of them formed into an Huygenian or Herschelian 

 eye-piece would, I think, be found to stop enough light to 

 enable the eye to bear the lustre of that body. Dr. WoUaston 

 probably never saw a garnet lens, and I know that he never 

 saw a dj^mond one, for he utterly disdained to inspect that 

 which was presented to him by Mr. Pritchard for examination. 

 His want of inquisitiveness in this respect was very remarkable, 

 inasmuch as a diamond lens, supposing it to be good for no- 

 thing, is still a curiosity, for many have disbeheved, and still 

 disbelieve to this day, the possibility of working a diamond 

 into a spherical curve. 



I conceive that the superiority of sapphire lenses is esta- 

 blished on a solid basis, and needs not the support of any 

 man's authority ; they will always (when properly made) be 

 able to stand on their own bottom, and maintain their 

 reputation now and hereafter. As an instance of Dr. W.'s 

 taste in them, I shall relate the following anecdote, which I 

 think too good to be lost. The facts I allude to are fresh, and 

 may be easily verified. Professor Shumacher wished to have 

 a sapphire lens ; Dr. Wollaston, being a man who knew dirt 

 from other things, was deputed to choose one for him, which 

 he did, out of a considerable number, presented to him by Mr. 

 Pritchard for that purpose. Having selected one, he marked 

 his initials upon the edge of it with a diamond, in order that 

 this identical lens, and no other, might be sent to Professor 

 Schumacher. It was sent accordingly, and the Professor re- 

 turned it as a bad one ; and bad enough it was allowed to be 



