The Microscope. 341 



if necessary, until the cement will dry hard in from two to five 

 hours ; if, at the end of two weeks, (in a warm room) it will 

 bear the test referred to above, it is sufficiently hard for filling 

 up around balsam mounts, which have become hard and solid ; 

 for labels it has no equal. 



Pour some of this cement into a 1 drachm phial and color it 

 sufficiently with a strong solution of black "Diamond" dye, 

 and you have a beautiful black cement. With " Diamond " dyes 

 you can give it any color. In this case, the acid must be left 

 out as it will precipitate the dyes and spoil the beauty of the 

 cement. Alcohol, 50 per cent., will have to be used in lieu of 

 the acid, and the mixture placed in warm water, if too cold or 

 thick. 



EDITOlCS,^ 



^^^ DEPARTMENT 



USUALLY the only immersion fluids at the microscopist's 

 command are water, cedar oil, and glycerine made dense 

 by dissolving in it either cadmium sulphate, zinc carbolate or 

 some other salt. With homogeneous immersion objectives, or 

 those using an immersion medium with a refractive index as 

 nearly as possible that of crown glass, so that the cover, the 

 immersion medium and the front lens may form one homogene- 

 ous combination, with these objectives, water of course cannot 

 be used, so that the microscopist must have resinated cedar oil 

 or the glycerine solutions just referred to. But to obtain the 

 best results from these first-class homogeneous immersion ©bjec- 

 tives, it is important that the immersion fluid should have the 

 proper refractive index, that of crown glass being L5, of cedar 

 oil 1.515, the glycerine fluids varying in a way that the micro- 

 scopist has usually no means of finding out. Prof. H. L. Smith 

 has devised a simple and successful little instrument for the 

 measuiing of the refractive index of such liquids, but, so far as 

 I know, it is not in the market. The microscopist must there- 

 fore rely on the optician that sometimes by accident plays him 

 false, and so deprives him of the best that his objectives can do. 



