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THE POPULAR SCIENCE MONTHLY. 



of the complementary color of the small square with the color of the 

 ground ; and, by recollecting this, we can easily retain this class of facts 

 in the memory. 



There is another similar experiment, which is simpler than those 

 just described, but which nevertheless is instructive : A small square 

 of black paper is to be placed on a sheet of red paper, and the atten- 

 tion in this case is to be directed to a mark on the edge of the former 

 {see Fig. 6). When the black square is suddenly removed, the observ- 

 er sees in place of it a more luminous spot, which in the case before 



Fig. 6.— Red Geound with Black Paper. 



Fig. 7.— Red Ground with Intense 

 Red Image. 



us will, of course, be red ; but what is remarkable is the circumstance 

 that this red image will be more intense or saturated in color than the 

 rest of the ground. The rest of the sheet of red paper will look as 

 though gray had been mixed with its color (Fig. 7). This experiment 

 will, of course, succeed with paper of any bright color, and Helm- 

 holtz has found that the same effect can be obtained with the pure 

 colors of the prismatic spectrum. The explanation, according to our 

 theory, runs about thus : While we are in the act of looking at the 

 edge of the black square, red light is passing into the eye, and is fa- 

 tiguing all those portions of the retina that are not protected by the 

 presence of the black square; it thus happens that the ability of the 

 larger portion of the retina to receive the sensation of red is consid- 

 erably diminished ; the ability of the protected portion, of course, 

 suffers meanwhile no such change. When the black square is sud- 

 denly removed, the unfatigued portion of the retina receives a power- 

 ful impulse from the red surface, but the effect produced upon the rest 

 of the retina is inferior in dearree. This accounts for the fact that the 

 image of the square is brighter or more luminous, and we can easily 

 understand why it is at the same time more intense or saturated in 

 color, if we remember, as explained in Chapter IX., that red light ex- 

 cites into action not only the red nerves, but to a lesser extent the 

 green and violet nerves. Now, as the red nerves begin to be fatigued, 

 the action of the other two sets will be relatively more powerful than 

 at first, so that gradually the sensations of green and violet begin to 

 add themselves to that of red, or, what is the same thing, the sensation 



