90 THE MICROSCOPIST. 



or stone will often indicate under what conditions the 

 rock was formed. Thus crystals with water cavities were 

 formed from solution ; those with stone or glass cavities 

 from igneous fusion ; those with both kinds by the com- 

 bined influence of highly heated water and melted rock 

 under great pressure; while those that contain no cavi- 

 ties were formed very slowly, or from the fusion of homo- 

 geneous substance. 



Use of Polarized Light. Mr. Sorby states that the 

 action of crystals on polarized light is due to their double 

 refraction, which depolarizes the polarized beam, and 

 gives rise to colors b} 7 interference if the crystal be not 

 too thick in proportion to the intensity of its power of 

 double refraction. This varies much, according to the 

 position in which the costal is cut, yet we may form a 

 general opinion, since it is the intensity and not the char- 

 acter of the depolarized light which varies according to 

 the position of the crystal. There are two axes at right 

 angles to each other, and when either of them is parallel 

 to the plane of polarization, the crystal has no depolariz- 

 ing action, and if the polarizing and analyzing prisms are 

 crossed, it looks black. On rotating the crystal or the 

 plane of polarization, the intensity of depolarizing action 

 increases until the axes are at 45, and then diminishes 

 till the other axis is in the plane. If, therefore, this takes 

 place uniformly over a specimen, we know that it has one 

 .simple crystalline structure, but if it breaks up into de- 

 tached parts, we know it is made up of a number of sepa- 

 rate crystalline portions. 



The definite order that may occur in the arrangement 

 of a number of crystals may indicate important differences. 

 Some round bodies, for example, like oolitic grains, have 

 been formed by crystals radiating from a common nu- 

 cleus ; whilst others, as in meteorites, have the structure 

 of round bodies which crystallized afterwards. 



Sir .D. Brewster discovered that many crystals have 



