LIGHT. 



131 



9. A bar of glass half an inch thick, an inch broad, 

 and six or eight inches long, may be gently bent with 

 the fingers while held in place for projection. The 

 strain induces double refraction, and that manifests 

 itself by bands of light or dark, or color. 



All of these should have their outline sharply pro- 

 jected by an objective of proper focal length. 



10. A small crystal of Iceland spar, having its obtuse 

 angles ground off and polished so as to present a sur- 

 face as much as a quarter of an inch square, will pre- 

 sent a beautiful series of rings and bars when projected. 



Fig, 97. 



It will not be necessary to use an objective, but 

 simply to put both crystal and prism in or so near to 

 the focus of the condenser that as much light as possi- 

 ble may be transmitted through them. When in place 

 let each in turn be rotated upon its axis, and observe 

 the appearance and disappearance of the light and dark 

 bands. At a distance of twenty feet from the prism the 

 outer rings should be about four feet in diameter, 



11. A crystal of rock candy, with parallel faces, and 

 not more than the twentieth of an inch in thickness, 

 will present another system of rings and bands. Pro- 

 ject it in the same manner as the spar was projected. 



12. A piece of a quartz crystal cut at right angles 

 to its axis will, if projected in the same way as the last, 

 exhibit colors upon the screen, which will vary as the 



