PEESIDENTIAL ADDRESS. 5 



beam of ordinary light not polarised in any particular way is pro- 

 pagated through it. 



To polarise a beam of ordinary light, then, it is necessary to 

 have some arrangement which will cause the vibrations of the ether 

 particles to be restricted to one plane by destroying any vibratory 

 motion they may have in a direction perpendicular to that plane. 



Fig. 2. — Nicol's Prism. 



Polarisation by Double Refraction.— If we take a piece of 

 Iceland spar and lay it over a piece of black paper in which a 

 small pinhole has been made, the pinhole appears doubled. If 

 the spar is laid on a piece of paper with writing on it, the writing 

 similarly appears doubled. This shows that the spar divides every 

 beam of light into two, which pass through it in different direc- 

 tions. The beams of light w^hich are thus separated by the crystal 

 are polarised in two perpendicular directions. If we can get rid 

 of one of the two beams, we shall thus be able to get a beam of 

 polarised light. 



This is done by cutting the crystal of spar in halves and then 

 joining the two halves together with Canada balsam (Fig. 2). 

 One of the two polarised beams in the crystal (called the ordi7iary 

 ray) strikes the balsam surface so obliquely that it cannot pass 

 through, but is totally reflected out of the way. The other beam 

 (called the extraordinary ray) strikes the surface less obliquely, 

 and most of it is transmitted through the balsam and emerges at 

 the opposite surface of the crystal. 



This arrangement is called a Nicer s prism, and the polariscope 

 of a microscope consists essentially of two Nicol's prisms — one 

 fixed below and the other fixed above the object to be examined. 

 The former is called the polariser and the latter is called the 

 a7ialyser. 



