no 



THE ART OF PROJECTING. 



focus of the lens, and then the light will appear as a 

 brilliant white spot. Interpose a piece of colored 



glass, and the spot will 



at once change its color. 



3d. By reflection from 



a concave mirror. The 



colored rays will be con- 



^^9* s^' verged as white light 



would be, and appear upon a small screen placed at the 



focus as a spot of white light 



4th. By reflection from a 



series of small mirrors. Let 



the spectrum fall upon the 



small mirrors, and so incline 



them that they will reflect Fig, 87. 



the light to the same place upon the screen or the wall. 



5th. By rotating colored disks. 



Disks painted with the colors of the spectrum are 



sold in the market under the name of Newton's disks. 



They are made by pasting sections of colored tissue 



paper upon a large, stiff pasteboard disk. 



These colors should have the following angular 

 value : — 



Red, 60°, 

 Orange, 35°, 

 Yellow, 55^ 

 Green, 60°, 



Blue, 55°, 

 Indigo, 35°, 

 Violet, 60°. 



This disk may be rotated upon the whirling table, 

 or, what is much better, a zoetrope rotator, and it will 

 appear a dusky white. It will be better to have a strong 

 light thrown upon it while it is turning. 



Another good way is to cut disks of properly-colored 

 papers and make a radial slit in them. When put 



