THE POLARIMETER 69 



more closely to consider what happens when a ray of light 

 passes through the crystalline medium, and for this purpose 

 we may study a crystal of calcspar. Calcspar crystallises in 

 beautiful rhombs which are colourless and transparent ; 

 unlike a rhomb of glass, however, we shall find that if a 

 crystal of calcspar is placed over an inkspot on a piece of 

 white paper two inkspots will be seen : this is known as the 

 phenomenon of double refraction. Calcspar, hke tourmahne, 

 belongs to a class of crystals whose density, or the packing of 

 whose particles, is difEerent in different directions. Now it 

 is a simple consequence of the undulatory theory of hght, that 

 the velocity of propagation of a wave varies according to the 

 density of the medium, and further that, owing to this altera- 

 tion of velocity as the wave passes from one medium to 

 another of differing density, alteration of the direction of the 

 wave takes place. As the density of the calcspar crystal is 

 different in different directions the rays vibrating along one axis 

 will emerge from the crystal in a direction differing somewhat 

 from those vibrating in the plane of the other axis ; thus we 

 have either two images in the case of the inkspot, or, if we 

 direct a ray of hght upon the face of the crystal, two beams 

 will emerge. A further phenomenon is observed; if we 

 slowly rotate the crystal over the inkspot one spot wiU be 

 found to maintain its position, the other moves round with 

 the crystal, and similarly with the two rays of hght. The 

 ray whose position remains unaltered as we move the crystal 

 is known as the ordinary ray, because it obeys the ordinary 

 laws of refraction, that is, a constant relation always obtains 

 between the angle of incidence and the angle of refraction. 

 This is not the case with the ray giving rise to the movable 

 image. This ray is therefore referred to as the extraordinary 

 ray. 



The pecuHarity of tourmahne is, that while it breaks up 

 the ray into two, in the same way as the rhomb of calcspar 

 does, the planes of vibration being hkewise at right angles 



