ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



509 



author finds that his ocular successfully determines all the above pro- 

 perties of a thin section. The apparatus is represented in fig. 59, and 

 consists essentially of a metal holder, which is inserted in the Microscope- 

 tube in place of the ordinary ocular, and into which in turn a positive 

 Ramsden ocular is introduced at A, and certain plates mounted in metal 

 carriages, a, b, c, are inserted at B. Cross hairs are attached to the base 

 of the tube A, and are practically in the same plane with the upper 



Fig. 60. 



surfaces of the sliding plates, a, b, c, with the result that on focusing 

 the Ramsden ocular on the cross-hairs, the divisions marked on the 

 plates, a, b, c, are also in focus, and their relative movements can be read 

 off directly. With the above arrangement the optical constants required 

 can be measured directly by means of the three plates. 



Plate a (figs. 60, 61) is for measurement of birefringence, and is a 

 combination quartz wedge 35*3 mm. long, and 10 mm. wide. It con- 



Fig. 61. 



sists (1) of a quartz wedge cut parallel with the principal axis (direction 

 of elongation = c) 0*5 mm. thick at the thin end, and 0'<s9 mm. at 

 the thick end, its pitch being, therefore, about 6° 16' ; and (2) of a 

 quartz plate with direction of elongation a of same length and width, 

 and 0*56 mm. thick. If these dimensions be followed exactly, T V mm. 

 divisions ruled on the upper surface of the wedge (fig. 61) will give 

 directly the difference in distance in /i/x, between emergent light waves 

 at a particular point. Thus, for sodium light the distance between 

 successive interference bands will be 5*89 mm. The zero line of the 

 scale must coincide precisely with the black line of exact compensation 

 between wedge and superimposed plate. In the present wedge this is 



