72 DOUBLE REFRACTION 



(91) We have seen that in Iceland spar the extraordi- 

 nary index is less than the ordinary, and that consequently 

 the extraordinary ray is refracted from the axis. This, how- 

 ever, is not universally true of all double-refracting crystals. 

 Biot discovered that there were many crystals in which the 

 extraordinary index was greater than the ordinary, and in 

 which, therefore, the extraordinary ray is refracted towards 

 the axis. Crystals of this kind he called attractive, while 

 those of the former were denominated repulsive. Among the 

 attractive, or (as they are sometimes called) positive crystals, 

 are quartz, ice, zircon ; the repulsive or negative class is far 

 more numerous, and includes, among others, Iceland spar, 

 sapphire, ruby, emerald, beryl, and tourmaline. 



The Huygenian law applies to attractive as well as to re- 

 pulsive crystals, it being observed, that in the former case the 

 axis of revolution of the ellipsoid must be the greater axis of 

 the generating ellipse ; or, in other words, that the spheroid 

 is prolate instead of oblate. 



(92) It has been hitherto assumed that there is but one 

 optic axis in every crystal, or one direction only along which 

 a ray will pass without division. It was reserved for Sir 

 David Brewster to discover that the greater number of crys- 

 tals possessed two optic axes. Among the most remarkable 

 of the crystals with two axes may be mentioned arragotiite, 

 mica, sulphate of barytes, sulphate of lime, topaz, and felspar. 

 The angles range in magnitude through the entire quadrant ; 

 and they accordingly afford a new and important criterion 

 for the distinction of mineral substances. 



(93) It appears from the foregoing, that crystalline bodies 

 may be divided into three classes, with respect to their action 

 upon light, namely 



I. Single-refracting crystal*. 



