2 THE POPULAR SCIENCE MONTHLY. 



squares are really identical in hue. By a somewhat analogous pro- 

 ceeding we can cause a surface which properly has no color of its own, 

 which is really gray, to appear tinted red, blue, green, etc. These changes, 

 and others of a like character, are produced by what is called contrast, 

 and are partly due to actual effects generated in the eye itself, and 

 partly to fluctuations in the judgment of the observer. The subject of 

 contrast is so important that it will be worth while to make a some- 

 what careful examination of the laws which govern it, and it will be 

 well for the reader to repeat some of the simple experiments described 

 below. If we place a small piece of bright-green paper on a sheet of 

 gray drawing-paper, in the manner indicated in Fig. 2, and then for 

 several seconds attentively look at the small #;ross in the centre of the 

 green slip, we shall find, on suddenly removing it, that in its place a 

 faint image of a rose-red color makes its appearance {see Fig. 3). This 



Fig. 2.— Gray Paper with Green Slip. 



Fig. 3. — Grat Paper with Rose-colored 

 Image. 



red image presently vanishes, and the gray paper resumes its natural 

 appearance. The rose-red ghost which is thus developed has a color 

 which is complementary to that which called it into existence, and this 

 will also be the case if we employ little squares of other colors : red 

 will give rise to a greenish-blue image, blue to a yellow, violet to a 

 greenish-yellow, etc., the color of the image being always complement- 

 ary to that which gave rise to it. Upon this account these images 

 are called negative, since as far as the color goes they are just the re- 

 verse of the images which are first presented to the eye of the observer. 

 They are also often spoken of, in older treatises on optics, as " the acci- 

 dental colors." It is quite easy to explain their production with the aid 

 of the theory of Young and Helmholtz. Let us take as an example 

 the experiment just described: According to our theory the green light 

 from the little squares of paper, acting on the eye, fatigues to some 

 extent the green nerves of the retina, the red and violet nerves mean- 

 while not being much affected. When the green paper is suddenly 

 jerked away by the string, gray light is presented to the fatigued reti- 

 na, and this gray light may be considered to consist, as far as we are 

 concerned, of red, green, and violet light. The red and violet nerves, not 

 being fatigued, respond powerfully to this stimulus ; the green nerves, 

 however, answer this new call on them more feebly, and in consequence 



