26 



THE AMERICAN MONTHLY 



[February, 



Immersion Fluids. 



[Dr. Henri Van Heurck has com- 

 municated to the " Society Beige de 

 Microscopie " an article on the for- 

 mulae of new liquids for homogene- 

 ous-immersion objectives, the follow- 

 ing summary of which has been pre- 

 pared from the Bulletin des Stances. 

 —Ed.] 



Personally, we have been able to 

 appreciate, perhaps better than 

 others, the importance of homogene- 

 ous-immersion objectives, for it is by 

 their use that the numerous drawings 

 of the Synopsis des diatomdes de Bel- 

 gique have been completed in a rela- 

 tively short time. When we think of 

 the trouble which the employment of 

 monochromatic light occasions, of the 

 frequent interruptions of work neces- 

 sitated by the absence of sunlight, we 

 cannot sufficiently congratulate our- 

 selves because of this happy discov- 

 ery, which has enabled us to advance 

 the publication of our work, perhaps 

 by several years, of which all the 

 drawings, both Mr. Grunow's and 

 our own, have been made with the 

 aid of objectives of this new system. 



At first homogeneous-immersion 

 objectives were constructed without 

 collar-adjustment. Is this well ? At 

 one time we believed so, but after 

 having used for a short time a ^ 

 homogeneous-immersion, by Tolles, 

 with adjustment, we have changed 

 our opinion, and found that the cor- 

 rection renders great service in cer- 

 tain cases ; for example, when the ob- 

 jective is to be used on microscopes 

 having tubes of different lengths, 

 which is sometimes necessary, as in 

 the case when one has to make draw- 

 ings of a definite magnification, and 

 also when the index of refraction of 

 the immersion fluid slightly changes 

 by extremes of heat or cold, or from 

 other causes. 



The oil of cedar, proposed as the 

 best immersion fluid, has the disad- 

 vantage to dissolve the varnish, and its 

 fluidity makes it inconvenient to use. 



Prof. Abbe has examined 300 sub- 



stances without finding one better 

 than oil of cedar. The researches re- 

 corded here were conducted by the 

 aid of Prof. Abbe's refractometre, and 

 have finally been crowned with suc- 

 cess ; since we now know a consider- 

 able number of substances which may 

 be advantageously employed. The 

 necessary conditions are : — 



1. A suitable index of refraction. 

 This index, for objectives now made, 

 is about 1.5 10. The crown-glass, of 

 which the cover-glass and the front 

 lenses of the objectives are made, has 

 an index from 1.5 10 to 1.520, meas- 

 ured by the F line of the spectrum. 



2. A dispersive power near as pos- 

 sible to that of crown, which is about 

 .0060 measured between the lines D 

 and F. 



3. It must not be excessively fluid. 



4. It must not attack the varnish of 

 the cells. 



In this note we give the results of 

 our researches, but first we pass in re- 

 view the liquids heretofore proposed. 



These liquids may be divided into 

 two classes : — 



1. Solutions of chemical products. 



2. Vegetable substances. 



All the substances proposed up to 

 the present time are solutions of salts 

 in glycerin. The best of these liquids 

 is Bassett's fluid, which is prepared 

 by dissolving crystals of chloral hy- 

 drate in glycerin. The principle in- 

 convenience of Bassett's fluid is its 

 action upon the varnish and especi- 

 ally shellac. This liquid has been al- 

 most discarded. The other solutions 

 are : — 



Cadmium chloride in glycerin ; in- 

 dex 1.504. 



Zincic iodide in glycerin ; index 

 1.507. dispersion 0.0080. 



Zinc sulphocarbolate in glycerin ; 

 index 1.500. 



Finally, distilled zincic chloride 

 has been proposed, but this is a dif- 

 ficult substance to manage. 



Among the vegetable substances 

 proposed up to the present time we 

 have : — 



Oil of cedar, formed by the distilla- 



