Lenses for Microscopes. 19 



peculiar appearance of an aggregation of minute shivery cris- 

 tallized flaws, sometimes radiated and sometimes in one direc- 

 tion, which can never be poHshed out : I beheve I could dis- 

 stinguish with certainty a bad lens from a good one by this 

 phenomenon without looking through it*. Precious stones, 

 from their crystalized texture, are liable to the same defects for 

 optical purposes as diamonds. 



Having ascertained the goodness of a stone it must next be pre- 

 pared 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 grinding 

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

 be possible to neutralize the naturally low spherical aberration 

 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 abra- 

 sion of its surface. When this is accomplished, a concave tool 

 of cast iron must be formed of the required curve in a lathe, 

 having a small mandril of about j^ths of an inch in diameter, 

 and a velocity of about 60 revolutions per second ! The dia- 

 mond must now be fixed by a strong hard cement (made of 

 equal parts of the best shell lac and pumice-stone powder, care- 

 fully melted together without burning) to a short handle, and 

 held by the fingers against the concave tool while revolving. 

 This tool must be paved by diamond povvder, hammered into it 

 by an hardened steel convex punch : vvhen the lens is uniformly 

 ground all over, very fine sifted diamond-dust carefully washed 

 in oil must be applied to another iron concave tool (I may here 

 remark, that of all the metals which I have used for this pur- 

 pose soft cast iron is decidedly to be preferred) : this tool must 



* As many amateurs of science might take an interest in the inspection 

 of the pecuhar etfect these lenses have on transmitted light, I shall be 

 happy to exhibit them, as also the pertfect lens. 



C 2 



