MOUNTING DIATOMS. 219 



oleaginous nature, and is soluble in alcohol. Its 

 density is 1*555, and refractive index 1'6. Its index 

 of visibility is about twice that of Canada balsam. 

 Mr. Stephenson, who first directed special attention to 

 the subject, came to the conclusion that the visibility 

 of lined objects depends upon the difference of the 

 refractive indices of the object observed and the medium 

 in which it is placed. 



Taking the refractive index of air as 1*0, and diato- 

 maceous silex as 1'43, the visibility may be .expressed 

 by the difference *43. 



The following table may be constructed : 



Refractive indices Visibility of sile.it 



(taken approximately). (Refr. index = T43). 



Water = 1'33 ... 10 



Canada balsam = 1'54 ... 11 



Bisulphide of carbon = 1'CS ... 25 



Sol. of sulphur in bisulph. ... = 1'75 ... 32 



phosphorus ,, ... = 2'11 ... 67 



These data relating to visibility must be taken in 

 connection with the numerical aperture 1 of the objec- 

 tives and of the illuminating pencil. The effect pro- 

 duced on diatoms is very remarkable, the markings on 

 their siliceous frustules being visible under much lower 

 powers. 



So that the visibility of the diatom mounted in 

 phosphorus as compared with balsam is as sixty-seven 

 to eleven ; in other words, the image is six times more 

 visible. Mr. Stephenson's phosphorus medium is 

 composed of a solution of solid or stick phosphorus 

 dissolved in bisulphide of carbon. Great care is re- 

 quired in preparing the solution owing to the very 

 inflammable nature of the materials. So small a 

 quantity of the bisulphide of carbon is required to 

 dissolve the phosphorus that the diatom may be said 

 to be mounted in nearly pure phosphorus. Remark- 

 able enough, this medium has the reverse effect upon 

 some other test-objects, as Podura and Lepisma scales, 

 which lose their characteristic markings. 



F. M. Rimmington's Glycerine Jelly is especially 



(1) Professor Abbe introduced a new expression for apertnre (i.e., "numer- 

 ical aperture"), by which the relative resolving power cf different objec- 

 tives is se*n by the reading of their numerical apertures. 



