ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 121 



Secure Method of Setting the Front Lens of Oil-immersion 

 Objectives. — Mr. J. Mayall, jimr., sends us the following note com- 

 municated to him by Mr. Wenham : — 



" The method usually employed to set the minute fronts of oil- 

 immersion lenses, has been to ' burnish them in,' that is, to fit them 

 exactly into the cell, leaving a thin rim of metal projecting round the 

 lens, and while the setting is running in the lathe laying this over the 

 edge of the glass, which should be slightly chamfered. With large 

 eye-piece lenses this is an easy and certain operation, but with lenses 

 less than one-eighth of an inch in diameter, it is one of the most 

 difficult and delicate oj)erations in optical manipulation. Not only is 

 there some risk of fracture, but if at the time that the metal grips 

 the glass, the surface should get out of truth from the tilting of the 

 lens, this cannot afterwards be forced to run true, as the pressure 

 required would certainly cause fracture ; the only remedy is to cut 

 out the lens and try again. I have seen a great number of minute 

 lenses, that have been ' burnished in,' running out of truth simply 

 because there is no absolute control over the operation ; and to make 

 an oil-tight fit, the brass, or other metal equally hard, must bear 

 heavily on to the fragile glass, with the liability of distorting its 

 figure. 



" Many years ago I set some minute eye-piece lenses in the following 

 manner : — I turned the cell clear out, so that the lens would drop 

 easily through. I then heated the cell, and with a conical-pointed 

 copper wire, well tinned, and a fragment of rosin for a flux, I twisted 

 the wire round till a ring of tin was well fused round the hole. I 

 then turned the tin lining out true, to form the cell and bed for the 

 lens, leaving the projecting ridge necessary for burnishing. This 

 operation was performed with an ivory stylet lubricated with moist 

 soap. Before the finishing touch is completed, if the face of the lens 

 is seen to run out of truth (ascertained by the usual ' candle ' test), 

 the soft metal will yield to a moderate pressure applied on the proper 

 side ; the burnishing may then be completed without fear of the lens 

 wabbling, and, finally, the edges neatly finished with a turning tool. 

 I have set the front lenses of objectives this way, and the soft tin 

 plies so well round the glass that no leakage occui'S with any kind of 

 oil or spirit ; and lenses thus mounted may be pushed out from the 

 back of the cell, for alteration or repair, without risk of fracture, as 

 the portion of tin that has been laid over is raised up again wdthout 

 the application of a dangerous degree of force. 



" The volatile oils or spirits used for immersion lenses, act ener- 

 getically in softening or dissolving either Canada balsam or shellac, 

 and it will not answer to employ these substances for rendering them 

 oil-tight. 



" For the purpose of making a leaky joint tight, I have found 

 ordinary sulphur answer perfectly, as it is not acted upon by any cold 

 immersion fluids that can be used, and as its melting point is about 

 220° Fahr., the heat required in its employment will not injure the 

 coat of lacquer on brasswork. In order to* cement a front lens oil- 

 tight in its cell, it is sufficient to place this, with lens fitted in 



