MICROSCOPICAL JOURNAL. 



23 



diatoms used were in alcohol. I first 

 placed the required quantity on the 

 inverted cover, dried them, added 

 sufficient medium to cover them, 

 heated the cover to drive the air out 

 of the cavities of the diatoms and 

 cause the fluid to enter, added, if 

 necessary, a little more, inverted in 

 place on the slide on a turn-table, 

 and removing any surplus by a blot- 

 ter, run a ring of balsam or shellac 

 cement around, thus finishing at one 

 operation. The anilin is not very 

 volatile, and the adhesion of the cover 

 very slight, but with care, using a 

 long bristled brush and thin balsam, 

 a coat can be got quite sufficient to 

 seal and fix the cover in place, and 

 additional coats may be given when 

 convenient. Anilin, according to 

 Storer, dissolves its own weight of 

 sulphur ; if heat is used it will become 

 supersaturated, and crystals will form 

 on the slide, which are very prettv of 

 themselves, but of course are not de- 

 sirable with other objects. 



As Gladstone and others have in- 

 dicated that high refractive power 

 accomjDanies complex molecular con- 

 stitution, it is probable the best sol- 

 vents for our purpose will be found 

 among the carbon compounds like 

 anilin, chinolin, etc. Except in the 

 hands of a few persons, the value of 

 these mediums for demonstrating 

 structure appears to have been over- 

 looked. Coarse diatoms in air are 

 opaque for all practical purposes, 

 their refractive index, air taken as 

 unity, being 1.434. This is also more 

 than water 1.33, in which they are 

 quite transparent, more so in dilute 

 sulphuric acid which may be concen- 

 trated to have the same index as the 

 diatom, which then becomes invis- 

 ible. Calling mediums of less re- 

 fractive power than tlie diatom nega- 

 tive or — mediums, we now change 

 our relations to mediums of greater 

 refractive power than the object 

 which may be called -|- or positive, 

 a large number of which are arranged 

 in the accompanying table. The 

 relative visibility of any object may 



be determined by taking the nu- 

 merical difference of its index 

 from that of the medium enclosing 

 it. Each convexity or elevation ap- 

 pears brighter when the tube is 

 raised in a — medium, and a concav- 

 ity brighter when it is depressed. 

 Exactly reverse effects follow if the 

 medium is -j-i foi' each portion rest- 

 ing in a depression of the object acts 

 like a lens to concentrate the light. 

 These facts may be used to differen- 

 tiate the chemical nature of sub- 

 stances, for calcareous bodies have a 

 higher index than silicious, some- 

 times higher than that of Canada 

 balsam, hence it would be possible 

 to have two surfaces exactly oppo- 

 site in shape as regards elevations 

 and depressions, but of different in- 

 dices, give the same appearance if 

 mounted in a medium of interme- 

 diate index. The beginner in this 

 kind of work will do well to take a 

 piece of the embossed sheet-metal, 

 novsr readily obtained, which corres- 

 ponds in surface with some diatoms, 

 but in w^hich the structure is large 

 enough to be unmistakably under- 

 stood, and study it by reflected light 

 with modification of the direction of 

 the light, focus, etc., and carefully 

 note peculiarities of appearance. 

 Some of the compounds mentioned 

 in the table have not yet been tried, 

 and as soon as I can prepare them I 

 will report upon their availability. 



Table of Refractive Indices for the D 

 Line, compiled by Prof W. H. Seaman. 



References:— (R.) Report Brit. Ass. 1839. (B.) Hei- 

 bliitter, 1885, No. 4. (S ) Smith Amer. M. Micr. 

 Journ. V, 162. (J.) Stephenson, Joiirn. R. Micr Soc. 

 (2), ii, -67. 



R. Air, I 



R. Water, 1-334 



B. Methyl alcohol, . . . i-33o 



R. Alcohol, Sp. Grav. 0.815, . 1-365 



B. Ethyl alcohol, .... 1.430 



J. Silex of diatoms, . . . 1-436 



B. Glycerin, pure, .... 1.461 



R. Turpentine, .... 1-474 



B. Benzol C,, Hg, . . . . 1.507 



Copaiva, * . . . . 1.50 



B. Styrolene C^ H^, . . . 1.531 



J. Canada balsam, . . . i-54 



Cedar oil and dammar,* . . 1.54 



R. Crown glass, . . . 1-559 



* Various sources. 



