66 DOUBLE REFRACTION. 



(85) In proceeding to the consideration of this law, we 

 must observe, in the first place, that there is a certain direc- 

 tion in every double-refracting crystal, along which if a ray 

 be transmitted, ,it is no longer divided. This line is called 

 the optic axis, and all the phenomena of double refraction are 

 related to it. There are, properly speaking, an infinite 

 number of such lines within the crystal, all parallel to one 

 another ; so that the optic axis is fixed, not in position, but 

 in direction only. It has been already mentioned that the 

 line connecting the obtuse solid angles of the rhombohedron 

 of Iceland spar is the axis of the crystal. Now if we conceive 

 a crystallized mass of this substance to be subdivided into its 

 elementary molecules, which are of this form, the axis of 

 each of these molecules will be an optic axis. The optic axis 

 of the crystallized mass, therefore, is a direction in space 

 parallel to the axes of the elementary molecules, or equally 

 inclined to the three faces containing the obtuse solid 

 angle. 



(86) All the phenomena of double refraction are symme- 

 trical round the optic axis. To see this, we have only to polish 

 an artificial face on the crystal, perpendicular to the optic axis, 

 and to mark the course of the refracted rays. We shall then 

 observe that, when the ray is incident perpendicularly on this 

 face, or in the direction of the axis, it undergoes no deviation 

 by refraction, and the ordinary and extraordinary rays coin- 

 cide ; that for every other incidence the ray is divided, the 

 refracted rays being both in the plane of incidence, and the 

 deviation of the extraordinary ray being less than that of the 

 ordinary. This deviation of the extraordinary ray, and there- 

 fore the ratio of the sines, is the same for all rays equally in- 

 clined to the axis, whatever be the azimuth of the plane of 

 incidence. But when the inclination of the ray to the optic 

 axis is varied, the ratio of the sines of incidence and refraction 



