ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



;y,<? 



The illuminating apparatus consists of a mirror (plane and concave 

 sides), polariser, iris diaphragm, and condenser, the last three (figs. 89, 

 90, 91), being mounted on a detachable angle-piece g, to which is 

 likewise attached the rack tg (not shown in fig. 87). Those parts 

 which are capable of being thrown out of action, viz. the hinged carrier 

 of the upper condenser C B, the upper condenser C with the slider S 



M 



Fig. 89. 



for the iris diaphragm J, and the lower condenser, are shown in both 

 positions. The polariser, the lower and upper condenser lenses, are 

 shown half in section and half as they appear when withdrawn from 

 their respective mounts. The polariser is a large Glan-Thompson 

 prism with a symmetrical field of polarisation of 30° ; and the author 

 fully describes the means provided for its adjustment. J is the iris 





3} 



UC 

 JZrr=^ 



m 



Fig. 90. 



Fig. 91. 



diaphragm, as followed by the lower condenser lens U C, which yield an 

 approximately parallel beam of light. Both are carried by the slider S. 

 The iris diaphragm is placed above the polariser so as to render it 

 possible to limit at will the pencil of plane-polarised light. It is used 

 in conjunction with the polariser, e.g. for determining after Becke's 

 method the difference of refraction in minerals, twin lamina?, etc. The 

 essentially novel feature of the illuminating apparatus is the upper 

 condenser C (fig. 89). This is so mounted that it may be tilted back 

 June 17th, 1908 2 c 



