Il8 THE ART OF PROJECTING. 



I can thus exhibit overlapping circles of brilliant 

 red, blue, and green light, which produce a perfect 

 white by their admixture ; while at the same time there 

 is seen the compound yellow produced by the union of 

 red and green, the purple arising from the red and 

 blue, and a color varying from grass green to sky blue 

 produced by the combination of the green and blue 

 light. This experiment has the advantage of exhibit- 

 ing at the same time the three primary colors, red, 

 green, and blue, the compound colors produced by 

 their mixture, their complimentary tints, and the syn- 

 thesis of white light." 



The flat cells mentioned are made by cutting thin 

 pieces of board to the desired shape, and cementing 

 pieces of window-glass on each side by means of pitch. 



INTERFERENCE SPECTRA IN REFLECTED LIGHT. 



Fig. 91. 



Let a beam of light about an inch in diameter fall 

 upon a thin piece of mica, M, distant eight or ten feet 

 from the porte lumiere. A part of the light will be 

 reflected, and in that may be placed a slit at Z, and a 

 lens O may project the slit in the ordinary way. At 

 the focus of the lens place a good prism so as to have 



