[ 128 ] 



ipolarieeb Xigbt anb ite HppUcatione to 



the fIDicroacope. 



Presidential Address by G. H. Bryan, M.A. 



Part II. 

 Plate VII. 



Doubly Refracting Polariscopic Objects. 



MOST microscopical polariscopic objects are polariscopic on 

 account of their possessing the property of double refrac- 

 tion, as described, for the case of Iceland spar, in the 

 first part of this paper. Such is the case with many chemical cry- 

 stals, thin films of selenite, certain woody tissues of plants, starch 

 grains, sections of hoof and horn, many animal and vegetable 

 hairs, and certain polyzoa. Crystals belonging to what is called 

 the "cubic" system — of which common salt is an excellent 

 example — are not doubly refracting and therefore not polariscopic. 

 Glass, when properly annealed and unstrained, is not polariscopic, 

 but it becomes polariscopic when it is strained by being subjected 

 to great pressure or tension in one direction. Badly annealed 

 glass sometimes becomes strained in the same way in the process 

 of cooling, and it is then polariscopic. For this reason a polari- 

 scope is largely used in glass-works for testing whether the glass is 

 properly annealed or not. Any articles which exhibit bright 

 patches when the polariscope is arranged to give a dark ground 

 are rejected. 



Conditions necessary for Polariscopic Effects.— If a ray of 

 ordinary unpolarised light fall on a section of a doubly refracting 

 crystal, such as Iceland spar or selenite, it is split into two com- 

 ponent rays, which are polarised in perpendicular directions, and 

 these rays travel through the section in different directions and at 

 different speeds. As in the case of Iceland spar, it will be con- 

 venient to call one of these rays the ordinary and the other the 

 extraordinary ray,* and we shall call the directions in which these 



* In many crystals, such as selenite, neither of the rays ought properly to 

 be called an ordinary ray, but for our purpose the use of the terms " ordinary 

 ray " and " extraordinary ray " will not be misleading. 



