420 



THE POPULAR SCIENCE MONTHLY. 



approaches green ; the light is white, with a slight tint of pink (Fig 4) 

 Now, in this experiment, I have obtained our white light by the 

 actual mixture on the screen of yellow and blue light, furnished by 

 the same two glasses which, a little while ago, when superimposed, 



Fig. 3. 



gave us green light. The apparatus is so contrived that the glasses 

 send toward the screen yellow and blue beams of light, but, before 

 traveling far from the lantern, these beams are caught by this large 

 crystal of cale-spar, and each, as you see, is duplicated. Let us pursue 

 this matter a little further, and, to facilitate our judgment of the tints, 

 I throw on the screen, near the colored squares, a circle of unaltered 

 white light, for comparison. Perhaps we have failed to produce green 

 light from the circumstance that our yellow squares were too bright ; 

 with a simple contrivance I can diminish their luminosity without al- 

 tering their tint, and the rate of diminution you can easily watch in 



Fig. 4. 



WHITE 



the uncombined yellow square. This apparatus is now acting, but 

 though under its influence the tint of the central square changes, pass- 

 ing from white by a series of gradations into blue, you see that it 

 manifests no tendency to become green. Restoring the yellow squares 

 to their original brightness, in the same way I gradually cut down the 

 brightness of the blue squares, and yet fail to generate any hue ap- 

 proaching green. 



A result like this ought to shake, if not entirely destroy, our confi- 

 dence in the old theory, but Helmholtz has pursued the investigation 

 still further, and has proved, in addition, that the union of the pure yel- 



