THE AMERICAN 



MONTHLY 



MICROSCOPICAL JOURNAL. 



Washington, D. C, Febkuary, 1886. 



Vol. YII. 



No. 2. 



Mounting Mediums with High Re- 

 fractive Indices.* 



BY PROF. WM. H. SEAMAN. 



In a recent box of the Postal Mi- 

 croscopical Club was a slide of Am- 

 phlpleura pellucida having sepa- 

 rate mounts, in balsam, storax, and 

 tolu. This diatom is well known as 

 a very interesting test-object, the 

 stria on which were first clearly re- 

 solved in 1 87 1, and played a very 

 prominent part as a test in the battle 

 of the lenses that raged for several 

 years subsequently. The different 

 mediums on this slide, and the marked 

 improvement in definition resulting 

 from the use of highly refracting sub- 

 stances, suggested that this part of 

 microscopical manipulation was de- 

 serving of more attention, now that 

 the fundamental principles of the 

 construction of objectives appear to 

 be definitely settled. 



On seaixhing for tables of refrac- 

 tive indices that would guide me in 

 the selection of promising compounds, 

 I found three deserving of note, the 

 first in the Annual Report of the 

 British Association for iS39,by Rev. 

 Baden Powell, which includes the 

 original Fraunhofer determinations 

 of glass, and about 50 oils, and solu- 

 tions of acids and alkalies. The 

 second is by Prof. Matthieson in Die 

 Central Zeitung f. Optik u. Mechanik, 

 18S2.I containing 170 compounds, 

 about 40 of which were reprinted in 

 Journ. Royal Alicr. Soc. (2), iii, 

 588. The third is a list by J. H. 



* Rt.ad at a meeting of the Washington Micro- 

 bcopical Society, January \-2., 1886. 



t Mny be found in the library of Johns Hopkins 

 University. 



Gladstone in Jottrti. Chem. Soc, 

 xlv, 341, reprinted in Wiedemann's 

 Beibldtter^ 1885, of about 120 car- 

 bon compounds. 



Such of these determinations as are 

 likely to prove useful to the micro- 

 scopist are added hereto calculated 

 for the line p. d, which however is 

 not quite in the position to give a 

 mean refractive index, which lies 

 nearer the line E. 



Canada balsam appears to have 

 been first used as a mounting medium 

 by a showman in 1840, and has been 

 more generally employed than any 

 other substance. The great variety 

 of other compounds proposed in the 

 past few years have been selected for 

 their coloring or preservative quali- 

 ties, and as a rule were inferior to 

 balsam in refractive power. 



Among the earliest work in this 

 i-elation may be found the essay by 

 Mr. Stephenson in Jourtt. R. Alter. 

 Soc. in 1880. p. 567, and Mon. Alter. 

 jfoztrfz., 1873, from which some of 

 the data used here are taken. Mr. 

 Stephenson added to the list solutions 

 of phosphorus and sulphur in car- 

 bon bisulphide, and a saturated solu- 

 tion of mercuric iodide in potassium 

 iodide. The tendency of the first to 

 take fire spontaneously was very 

 ingeniously avoided by taking a short, 

 small vial without a neck, and fitting 

 a wooden former to it. A piece of 

 filter paper is pressed by the foi'mer 

 nearly to the bottom of the vial and 

 saturated with carbon bisulphide, and 

 a small piece of phosphorus laid in 

 the paper cup thus made. Solution 

 takes place rapidly, and by pressing 

 the former into the cup the liquid is 



