l886.] NEW-YORK MICROSCOPICAL SOCIETY. 75 



HIGH-REFRACTIVE MEDIA. 



BY PROF. H. L. SMITH. 

 {Read April id, i8S6.) 



The results of experiments made subsequently to the discovery 

 of the boro-glyceride and antimony-bromide medium, described 

 in a preceding paper,^ are of importance, and demand a brief 

 notice. The antimony compound works very pleasantly, and 

 still appears to be the best when high-refractive power is re- 

 quired ; but unless all excess is completely removed from out- 

 side the cover, it stains the protecting ring. The litharge and 

 gold-size ring and the zinc-white ring are merely darkened ; but 

 the black asphalt ring is softened. Thoroughly cleaning off the 

 excess around the cover remedies this difficulty. 



The chief improvement I would make in the formula given, I 

 now think, is the substituting of stannous chloride for antimony 

 bromide, and of arsenious acid for boracic acid. 



I find that a compound of stannous chloride, arsenious acid 

 and glycerine is so very slightly deliquescent, that the mounts 

 may be left for weeks without cleaning off the excess, and that 

 very little if any softening of the material ensues. The mounts 

 are easily cleaned, as the cover is very firmly attached. 



The medium is not so liable to turn when heat is applied, as 

 when boro-glyceride or gelatine and glycerine is used ; the lat- 

 ter, indeed, for that reason, is quite objectionable. The re- 

 fractive power of the mixture is not quite so high as when anti- 

 mony bromide is employed ; but the refractive power is quite 

 high enough for anything except the most hyaline tests ; and 

 as a little excess of material outside the cover does not discolor 

 the ring, and does not seem to alter by quite long standing with- 

 out a ring, I now prefer this compound. 



This medium is prepared as follows : Weigh out six parts of 

 stannous chloride, and two to two and one-fourth parts of pure 

 arsenious acid. Melt the stannous chloride in a test tube and 

 boil it for a little while; add while hot an amount of glycerine 

 equal to the bulk of the melted stannous chloride, not more ; heat 

 and shake until it forms a perfectly clear solution. Add now, 

 little by little, the arsenious acid, constantly shaking and heating 



^See this Journal, antCy p, 13, 



