22 



THE AMERICAN MONTHLY 



[February, 



forced through the filter into the bot- 

 tom of the viaL Having the objects 

 and cover in place on the slide, a 

 rapid transfer of a small quantity of 

 fluid to the edge of the cover is 

 effected by a pipette, and the fluid is 

 at once drawn under by capillary at- 

 traction. Success depends upon very 

 little exposure to the air, so as to 

 avoid oxidation. Mr. Stephenson 

 exhibited slides nine years old which 

 showed that if properly sealed such 

 preparations would be permanent. 

 The sealing may be eftected by glue 

 first and then a coating of balsam or 

 other cement, as carbon bisulphide 

 attacks all oils and resins. It does 

 not appear that Mr. Stephenson was 

 able to obtain so high refractive 

 power with sulphur solution as with 

 phosphorus, as he gives the index of 

 the first as 1.75, the second as 3.10.* 

 This may arise from the inferior sol- 

 ubility of sulphur. 



On mixing saturated solutions of 

 mercuric iodide and potassium iodide 

 and filtering from the residue an 

 aqueous fluid results with index of 

 1.68. This has the advantage that it 

 may be diluted with water to any 

 desired refractive power. 



Monobromo naphthalin, so far as I 

 know, was first used by E. Weiss- 

 flog, of Dresden, some of whose 

 slides were shown at a meeting of the 

 Royal Microscopical Society in 1S80, 

 and has been employed for some time 

 by Moller for preparing diatoms for 

 sale, but has a lower index than the 

 iodine mixture above mentioned. f 



Prof. Van Heurck (this JoitrnaL 

 Feb., 1882) has suggested white 

 vaseline 7 paits, copaiva 30 parts, as 

 a fluid for immersion lenses. This 

 makes a thick liquid remaining 

 where it is placed, indifl'erent chem- 

 ically and apparently adapted for a 

 mounting medium. J 



Finally, Prof. H. L. Smith has 

 recently! described the use of stan- 

 nous chloride and realgar, the latter 



*See also this J turnal, vi, 6. 



fyouin. A'. A'Jicr. Soc. (2), i, 151. 



J This yournal, vi, 86, for use of storax. 



\ This Journal, vi, 162, for high refracting media. 



a substance of higher index than any 

 before used. These articles are so 

 recent it seems onlv necessary to refer 

 to them here. 



On considering these various media , 

 and the substances not used, which 

 seemed likely to give good results, it 

 appeared singular that oil of cassia 

 had not been more thoroughly tried. 

 Its use has been suggested,* but I 

 have not observed any descriptions of 

 mounts made with it. The refraction 

 index being equal to that of carbon 

 bisulphide, recommended it to my 

 attention, and as all the essential oils 

 dissolve more or less phosphorus, I 

 made a saturated solution of phos- 

 phorus in oil of cassia, and with it 

 prepared several mounts, which, to- 

 gether with others in the mediums I 

 have described, are on the table for 

 your examination. This mixture is 

 easier to use because less inflammable 

 than carbon bisulphide, but contains 

 less phosphorus, as the latter is not 

 perfectlv soluble in oil of cassia as 

 in carbon bisulphide. I make a ring 

 of liquid glue on the slide, allow it 

 to dry, drying the diatoms on the 

 cover, addingthe solution, and quick- 

 Iv inverting the cover in its place, 

 then removing the surplus squeezed 

 out by blotting-paper, carefully press- 

 ing down on the glue ring, and then 

 sealing with balsam. The solution 

 smokes on exposure to the air, but in 

 these preparations there is no evi- 

 dence of acid flakes. 



On endeavoring to make a good 

 solution of sulphur in carbon bisul- 

 phide, it did not appear that suflicient 

 dissolved to get the full benefit of the 

 high index of sulphur. I immedi- 

 ately sought a better solvent, and 

 found it in anilin. 



On making a test mount of this 

 mixture on the mixed diatomaceous 

 material used for all the other me- 

 diums, I was surprised at the bril- 

 liancy and sharpness of definition, in 

 which, so far as I can judge, it excels 

 any other medium yet tried. The 



* Dippel, Das Mikroskop and Journ. R. Micr. Soc. 

 1880, p. 1044. 



