128 THE ART OF PROJECTING. 



with such small ones as come with the polarizing 

 attachment to common microscopes. With one having 

 a face three-quarters of an inch upon a side, everything 

 essential can be shown to a large audience. 



1. Place the prism at the focus of the lens so that 

 all the light will pass through it. Now, if the prism be 

 rotated upon the beam as an axis, the disk of light upon 

 the screen will decrease in brightness until it is nearly 

 or quite invisible; and if the prism be turned still 

 further in the same direction the light will reappear 

 and attain its maximum brightness when the prism 

 has been turned ninety degrees from the position 

 where the light disappeared. 



2. Turn the prism so that the light is cut off from 

 the screen; and then, holding it in that position, 

 slowly introduce a thin sheet of clear mica between 

 the lens and the prism. The light will reappear upon 

 the screen from that transmitted by the mica. If the 

 mica is as thin as the fiftieth of an inch, or less, the 

 light may be colored a beautiful blue or green or red. 

 Turn the mica round in its own plane, and these colors 

 will appear in succession. Let the prism be rotated 

 while the mica plate is held still, the same effects will 

 be observed. 



3. In the same manner experiments with thin plates 

 of selenite may be tried. 



4. Bring the lens forward so as to use it as an objec- 

 tive, and project a thin piece of selenite or of mica with 

 varying thicknesses. Hold the prism in the focus as 

 before. With each different thickness of the plates 

 different colors will be transmitted which are often 

 very beautiful indeed. If the pieces of these minerals 

 are not more than an inch square, a larger lens may be 

 used for a condenser, and then, with an objective of 



