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THE POPULAR SCIENCE MONTHLY. 



a disk cut like the last with open spaces, and armed with red, yel- 

 low, and blue glasses. You can predict the result beforehand: it 

 must be white light with added red light — and, as you see, we act- 

 ually do obtain a broad circular band of red light. Replacing this 

 disk by one provided with glasses capable of transmitting red, green, 



Fig. 5. 



and violet light, we find that their mixture actually gives us white 

 light. In all these experiments we have been content with the col- 

 ored light furnished by stained glasses, but Helmholtz has pushed 

 the investigation much further, and has obtained corresponding re- 

 sults by the use of the pure colored rays of the spectrum. 



I called your attention some time ago to the typical mode of 

 expressing the old theory by three intersecting circles of red, yellow, 

 and blue ; we have now again on the screen three intersecting cir- 

 cles ; the colors are red, green, and violet, with white at the centre 



Fig. 6. 



(Fig. 6). It expresses in a condensed form some of the main points 

 of the theory of Young and Helmholtz, and gives us at the same 

 time some of the chief laws of Nature's palette, showing, in a kind 

 of short-hand way, the changes which the tints of surfaces undergo 



