130 THE ART OF PROJECTING. 



color. If it does not, it is because the part removed 

 had the same thickness as one of the others, or differ- 

 ent from it by a wave length. Designs of any kind 

 that fancy may dictate may be thus made upon sheets 

 of mica. To project them plainly use an objective, as 

 in 4, and place the Nicols prism in the focus of it. 



Designs in selenite are still handsomer, and figures 

 of birds, butterflies, flowers, and fruits may be bought 

 in the market. Selenite is so brittle that a good deal 

 of skill is needed to work it, and it would be tedious 

 to a beginner. Such designs had better be purchased. 



6. Unannealed pieces of glass when they have a 

 regular form, as a square, a triangle, or a circle, make 

 good objects to project by polarized light. They are 

 generally a quarter of an inch or more in thickness, and 

 an inch or two in diameter. Pictures of their appear- 

 ance are often figured in works on physics. Pieces 

 of thick glass, fragments of glass vessels, and glass 

 stoppers of bottles often show double refractive power. 



7. A good way to exhibit the development of this 

 double refraction in glass is to take a piece of thick, 

 plain glass and stand its edge upon a piece of iron, 

 heated to redness, projecting the whole in the polar- 

 ized beam with the prism in its place. As the glass is 

 heated and strained, colors will develop upon the screen 

 and arrange themselves symmetrically, depending 

 wholly upon the external form of the glass. 



8 It is convenient to have a piece of glass as much 

 as a quarter of an inch in thickness and an inch 

 square or more, that is annealed, and consequently 

 gives no bands or colors. If this is strained by being 

 pinched in a hand-vice, tufts of light or black brushes 

 will be seen to start out from the place of pressure if 

 the whole be projected. 



