136 THE MICROSCOPE. Sept. 



is all that is needed. To remove the glycerine when 

 used by itself or in combination with a salt, as in the 

 homogeneous-immersion fluids, I am accustomed to 

 wipe away the greater portion with the Japanese paper, 

 and to remove the rest with a few touches of the tongue, 

 finishing with the dry paper. 



The cedar-oil used with homogeneous-immersion 

 lenses is thickened with dammar, so that a touch of the 

 moist tongae is likely to cause a deposit of some of the 

 gum on the lens, and to necessitate repeated applica- 

 tions of the paper moistened with alcohol to remove it."^ 

 It is better with this liquid to employ the paper alone, 

 and to finish with another pi^ce moistened with alcohol, 

 and to wipe the lens dry and perfectly clean with still 

 another piece. 



It is always well to clean an immersion-objective as 

 soon as possible after using it. I have known instances 

 in which the front lens was so insecurely burnished into 

 the metal cell that the fluid has penetrated to the back 

 of the glass, and made necessary a journey to the manu- 

 facturer. When the objective is to be out of use for 

 only a short time, it should be placed on the table with 

 the glass surface upward, and when the evening's work 

 is finished, it should be carefully returned to its brass 

 box, after a scrupulously neat cleaning. 



When about to measure the angular aperture of an 

 immersion-objective the front must of course be im- 

 mersed in the proper medium. This may be done by 

 applying a thin cover-glass to the front lens by means 

 of a drop of its special fluid. It is better however, to 

 estimate the numerical aperture by the method already 

 described. 



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* Since this was written I have received from the Bausch & Lomb Optical 

 Company, a supply of cedar-oil not open to this objection. 



