ZOOLOGY AND BOTANY, MICROSCOPY. ETC. 



719 



the new kinds of glass and fluorite. It has a N.A. of 0-95, an aplanatic 

 cone of 0-90; and the back combination a clear diameter of 0-45. 



Fig. 190. 



Fig. 191. 



Fig. 192. 



Polarised Light without Iceland Spar.* — Mr. J. S. Cheyney has 

 arranged a method of avoiding the use of Iceland spar, thus getting rid 

 of the main item of cost in a large polariscope : yet his beam of light is 

 brilliant, perfectly polarised in any plane, and of any diameter required. 



Fig. 193 gives a sectional view of his apparatus. The rays from 

 any strong source of light, such as the electric arc or sunlight, are 

 converged by a condenser to the diameter of beam required, and then 



Fig. 193. 



rendered parallel, as usual, by a concave lens a. The parallelised beam 

 is received either by a compound totally reflecting prism, with two 

 totally reflecting surfaces, b, b, or by two right-angled totally reflecting 

 prisms similarly placed ; or by two silvered mirrors set at 45° to the 

 direction of the beam. These carry it forward ; the two former with- 

 out loss, the latter with a slight loss from the successive reflections, to 

 impinge upon a silver mirror c, c, so set as to bring it to the proper 

 angle for complete polarisation by reflection from a bundle of plates of 



Micr. Bull., June 1900, p. 19 (1 fig.). 



3 C 2 



