THE MICROSCOPE. 27 



Instruments respecting the value of the precious stones for 

 single microscopes; and having seen their ftJl force, it was 

 agreed that they should undertake to grind a diamond into 

 a magnifier. The first diamond operated on was a small 

 brilliant, and it was proposed to give it the curves that in 

 glass would produce a lens of a twentieth of an inch focus. 

 " This stone, when nearly finished," says Mr, Pritchard, " fate 

 decreed that I should lose,* but having proved the possibility 

 of working lenses of adamant, I set about another, and selected 

 a rose diamond, in order to form a planoconvex lens." After 

 great labour and expense, this Mr. Pritchard so far accom- 

 plished, that on the 1st of December, 1824, he states, "he had 

 the pleasure of first looking through a diamond microscope." 

 Dr. Goring, who tried its performance on various objects, 

 both as a single microscope and as an objective of a com- 

 pound, was well satisfied with its superiority over other forms 

 of lenses. But here Mr. Pritchard's labours did not end, he 

 subsequently found that this stone had many flaws in it, 

 which led him to abandon' the idea of finishing it. Having 

 been prevented from resuming his operations on this refractory 

 material for about a year, Mr. Pritchard, in his third attempt, 

 met with another unexpected defect; he found that some 

 lenses, unlike the first, gave a double or triple image, instead 

 of a single one, in consequence of some of their parts being 

 either harder or softer than others. These defects were after- 

 wards found to be due to polarisation. Mr. Pritchard having 

 learnt how to decide whether a diamond is fit for a magnifier 

 or not, subsequently succeeded in making two planoconvex 

 lenses of adamant, whose structure was quite perfect for 

 microscopic purposes. " One of these," he tells us, " of one 

 twentieth of an inch in focal length, is now in the possession of 

 his Grace the Duke of Buckingham ; the other, of one-thirtieth 

 of an inch focus, is in his own hands." 



" In consequence of the high refracting power of a diamond 

 lens over that of glass, a lens of the former material may be 

 at least one-third as thin as that of the latter, and if the focal 



* Those who would wish to enter more in detail into this matter, 

 are referred to Pritchard's Microscopic Cabinet, p. 108. 



