39S 



PHYSIOLOGICAL PHYSICS. ichap. xxx. 



move round the other which is stationary. This 

 phenomenon is due to double refraction, and was dis- 

 covered in 1669 by the Professor of Geometry in 

 Copenhagen, Erasmus Bartholinus. The explana- 

 tion is that when a ray of light enters such a crystal 

 it is split up into two, and the two rays travel through 

 the crystal, with different velocities. One ray is 

 retarded more than another, that ray is, consequently, 

 refracted more than the other, and when the rays 

 issue from the crystal they do not unite, but are dis- 

 placed from one another, so that a double image is pro- 

 duced (Fig. 178). One ray travels through the crystal 



just as it would 

 do through a 



o 



plate of glass, 

 being refract e d 

 in the ordinary 

 way. This is the 

 ordinary ray, 

 and is the ray 

 which gives the 

 stationary image. 



* o 



The other ray, 

 which suffers 



the smaller degree of retardation, is called the extra- 

 ordinary ray, and is the ray which gives the movable 

 image when the crystal is rotated. To this ray the or- 

 dinary laws of refraction do not apply. Both rays are 

 of equal brilliancy. An explanation of the different 

 course of the two rays is offered by supposing 

 that doubly refractive crystals are not equally 

 elastic in all directions, and consequently vibrations 

 in different directions are subject to differences in 

 retardation. There is, however, always one direction 

 in which a ray of light will be transmitted without 

 double refraction. This direction is that of the optic- 

 axis of the crystal. Crystals that have more than 



Fig. 178. Iceland Spar. 



