1066 SUMMAKY OF CURRENT RESEARCHES RELATING TO 



very heavy and unsuitable for use with Microscopes to which it was 

 not specially adapted. The latest modification of form is that of 

 Mr. J. Grunow, shown in fig. 238.* 



Fig. 238. 



Device for Testing Refractive Index.f — A new device for testing 

 the refractive index of immersion media, and indicating how near an 

 approach to homogeneity with crown glass can be made, was described 

 at the recent meeting of the American Society of Microscopists by 

 Prof. H. L. Smith, who claims for this simple device superiority, 

 both as to ease of manipulation and accuracy of indication, over the 

 well-known wedge and bottle furnished by Herr Zeiss. In testing 

 any medium for immersion purposes, but little more than a drop of 

 the liquid is required, and the slightest variations of refractive index 

 are indicated by a considerable latitude of motion, vs^hen, in the 

 ordinary use of the wedge, these variations would be inappreciable. 



The instrument is used upon the Microscope, and a reference to 

 fig. 239 will make its application plain. A is an adapter about 



3/4 in. in length, with the Society 

 Fig. 239. screw outside and inside. This is 



attached to the Microscope, and 

 carries a 1 in. objective, a and h 

 w are two slips of crown glass, as 

 ^ near the refractive index of the 

 cover-glass as possible, 2 in. long 

 and 1/2 in. wide, each about 

 1/40 in. in thickness. In one of 

 these, h, near the end, a concave is ground to a depth of about one- 

 third or more of the thickness of the glass, and polished. 



To test whether a medium has the same refractive index as the 

 glass, and also the dispersion, a drop of it is put into the concave, and 

 the two slips of glass are placed together and inserted into an opening 



i * See Amer. Mou. Micr. Journ., vi. (1885) p. 183 (1 fig.). 

 t Ibid., PP.-181-2 (1 fig.). 



