PETROGRAPHY AND METALLOGRAPHY. 15S 



angle of the crystals can often be measured with a micro- 

 scope provided with a revolving circular stage, such as 

 should be used for all petrographical work. The inter- 

 section of cross-hairs placed in the eyepiece is adjusted 

 to coincide with the vertex of an angle, and one of the 

 cross-hairs is made to coincide with one side of the crystal; 

 the stage is then rotated until the adjacent side lies along 

 the cross-hair and the angle of rotation measured on the 

 graduated edge of the stage. Incomplete crystals without 

 definite outlines, corroded crystals affected by the molten 

 magma, broken or strained crystals, and various types 

 of incipient crystals may be made out. Finally no pe- 

 trographic examination is complete without the study 

 of polarizing properties. Before going further, therefore, 

 it is necessary to describe the micropolariscope and its 

 application. 



2. The Micropolariscope. — Ordinary light is made 

 up of vibrations of ether in all possible directions 

 which he in a plane at right angles to the direction of 

 transmission of the light-ray. The path of an individual 

 particle of ether in ordinary light would therefore be 

 constantly changing. Certain substances offer resistance 

 to the passage of such a light-ray, and confine its vibra- 

 tions to two planes at right angles to each other. Light 

 which, like that of the two resultant rays, vibrates only 

 in one plane, is known as plane polarized light. The 

 two rays produced, with vibrations in planes at right 

 angles to each other, obey different laws of refraction. 

 Thick layers of doubly refracting bodies like calcite cause 

 so wide a separation of the two rays as to produce two 



