486 JAMES CLERK MAXWELL. 



the direction in which the elasticity is greatest (of all the 

 directions which it can select), and the other in that in 

 which the elasticity is least. These rays, being transmitted 

 with different velocities, will be differently refracted, and part 

 company in the crystal ; and since in each ray the vibrations 

 are in one direction only, each ray will be plane polarised, 

 the vibrations taking place in the two rays in planes per- 

 pendicular to one another. 



A Nicol's prism consists of a long prism of Iceland spar 

 cut into two along a diagonal plane, the segments being 

 cemented together again by Canada balsam. The effect of 

 the balsam is to reflect one of the polarised rays out at the 

 side of the prism while it allows the other to pass through. 

 Consequently, if common light fall on a Mcol's prism, only 

 half of it will pass through, and this will be plane-polarised 

 in a particular plane. If another prism be placed in the 

 path of the ray and be situated similarly to the first, the 

 light will pass through it, but if it be turned around its 

 axis through a right angle, the light falling on it corresponds 

 to that which is reflected out of the prism by the balsam, 

 and no light passes through the second prism. Under these 

 circumstances the Mcols are said to be crossed. Such an 

 arrangement of two Mcol's prisms or any equivalent arrange- 

 ment which may be made with glass reflectors or crystals of 

 tourmaline, constitutes a polariscope ; the first Nicol is called 

 the polariser, the second the analyser. 



Fresnel first pointed out that by mechanical stress it is 

 possible to impart to glass a property analogous to that of 

 doubly refracting crystals, making its elasticity (for light) 

 different in different directions. Sir David Brewster showed 

 that a piece of glass heated and then suddenly cooled, i.e. 

 unanneale^, "possesses similar properties. If it be placed 

 between the polariser and analyser of a polariscope and 

 examined by white light, a gorgeous display of colour is 

 observed. The reason is that the plane-polarised light from 

 the polariser on entering the glass is generally split into 

 two rays vibrating in planes at right angles to one another, 

 and these pass through the glass with different velocities. 



