PETROGRAPHY AND METALLOGRAPHY. 155 
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 lie 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 cther, 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 twa 
