into Lenses for Microscopes. 151 



monds give a double, or even a species of treble refraction, form- 

 ing two or three images of an object. This property of course to- 

 tally unfits them for making lenses. I need not observe, that it 

 must be chosen of the finest water, and free from all visible flaws 

 when examined by a deep magnifier. It was extremely fortunate 

 for diamond lenses, that this substance is free from the defect 

 of double vision, otherwise diamonds en masse, might at once 

 have been abandoned as unfit for optical purposes. 



The cause why some stones give single vision, and others 

 several peculiar refractions, may also arise from different de- 

 grees of density or hardness occurring in the same stone.* Dia- 

 mond cutters are in the habit of designating stones male and 

 female ; sometimes a he and a she, as they have it, are united 

 in the same gem. Their he means merely a hard stone, and their 

 she a soft one. When a diamond which gives several refractions, 

 is ground into a spherical figure, and partially polished, it is seen 

 by the microscope to exhibit a peculiar appearance of minute 

 shivering crystallized flaws, sometimes radiated, and sometimes 

 in one direction, which can never be polished out. I believe I 

 could distinguish with certainty a bad lens from a good one 

 without looking through it. Precious stones from their crys- 

 talline texture are liable to the same defects for optical purposes 

 as diamonds. 



Having ascertained the goodness of a stone it must next be 

 prepared for grinding. It will in many cases be advisable to 

 make diamond lenses plano-convex, both because this figure gives 

 a very low aberration, and because it saves the trouble of grind- 

 ing one side of the stone. It must never be forgotten that it 

 may be possible to neutralize the naturally low spherical aber- 

 ration of a diamond lens by giving it an improper figure, or by 

 the injudicious position of its sides in relation to the radiant. 

 When the lens is to be plano-convex, cause the flat side to be 

 polished as truly plane as possible, without ribs or scratches ; 

 for this purpose the diamond should be so set as to possess the 

 capability of being turned round, that the proper direction with 

 respect to the laminae may be obtained. When the flat side is 

 completed, let the other side be worked against another dia- 

 mond, so as to be brought into a spherical figure by the abrasion 



• See Edinburgh Transactions , vol. viii. p, 160.— Ed. 



