4 i8 THE POPULAR SCIENCE MONTHLY. 



the yellow, and violet where it overlies the blue, the yellow and blue 

 giving a bright green, while the central space, under the action of all 

 the colors, is white (Fig. 1). As I said, the diagram now on the screen 

 is the typical expression of the old theory, and is constructed so as to 

 humor as much as possible the ideas of its supporters. If I had se- 

 lected three pigments, and honestly worked the diagram out by their 

 mixture, the result would have been much less brilliant and attractive. 



Fig. 1. 



Let me make an actual experiment on this point : I throw upon 

 the screen the image of three plates of stained glass ; their colors are 

 red, yellow, and blue ; they are also rich and intense. These pieces 

 are arranged so as to correspond to our three colored circles, and, in 

 fact, where the blue crosses the yellow, a green hue is actually pro- 

 duced, but it is darker than either of its constituents ; the violet is 

 much darker than the red or blue, and, where all the plates cross at 

 the centre, we have, instead of white light, complete darkness (Fig. 

 2). These peculiar strides in each case toward blackness would have 

 been observed, if a corresponding experiment had been made with any 

 three pigments, but this at present is a minor matter, and I leave it 

 for the consideration of a vastly greater difficulty under which the old 

 theory labors. 



Let us inquire how the superimposed yellow and blue glasses came 

 to produce green. The white light of the lantern contains all the dif- 

 ferent luminous waves, and it so happens that the yellow and blue 

 glasses both agree mainly in transmitting only waves of a medium 

 length, or, what is the same thing, green light. This can be proved 

 by an examination with the spectroscope, which also reveals the fact 

 that their agreement in this respect is by no means perfect, and that 

 the green rays are also compelled to pay toll for their passage, though 

 in a less proportion than the others. Exactly the same reasoning ap- 

 plies to blue and yellow pigments, and, from the effects produced by 

 their mixture, it does not in the least follow that yellow and blue 

 light make green light. This important point I now propose to test 

 by what may be considered a fundamental experiment. For this pur- 





