Nineteenth Annual Meeting. 



115 



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ner in which the missing components are destroyed: when by absorption or inter- 

 ference before the ray enters the eye, the color is said to be objective; when within 

 the eye, by the failure of certain portions of the optic nerve to perform their 

 function, we term the thereby-modified sensation conveyed to the brain a subjective 

 color. The influence of different portions of the spectrum upon these three sets of 

 nerves, has been studied by Helmholtz,^ Maxwell,^ and others, and the result of their 

 very laborious investigation is shown in the accompanying diagram. ( Fig. 1.) 



Each wave-length of the visible spectrum affects all three nerves, although in very 

 different degrees. And so all colors, even to the tints of the "pure" spectrum, are 

 really trifold. consisting of a mixture of these three primary color sensations. When 

 all three nerves are equally affected, the result is white; and the color differs from 

 white whenever the impression upon one set of nerves is enfeebled or unduly strength- 

 ened. 



These color nerves in the eye become temporarily weakened whenever they are 

 strongly stimulated, and their temporary enfeeblement is the chief cause of sub- 

 jective color. Contrast effects are among the most familiar examples of subjective 

 color sensation. They are 



always present, serving to ,-'' "^^^ 



heighten or to diminish ^'' ""---^^^ 



brilliancy of hue, accord- 

 ing to the arrangement of 

 the colors upon which we 

 gaze. If you will stave in- 

 tently for a moment upon 

 that patch of yellow light 

 upon the screen, you will 

 see that when I withdraw 

 the bit of glass which caused 

 it, from the field of the lan- 

 tern, that portion of the 



screen, really a pure white, becomes sufifused with an indescribably delicate shade of 

 blue. The blue is produced by white light from the screen entering your eyes, the 

 red and green-transmitting nerves of which have been somewhat fatigued by expo- 

 sure to yellow, which is principally made up of these two compounds. The nerves 

 which transmit violet have been resting, meantime, because those portions of the 

 lantern's rays which would have affected them were absorbed by the yellow glass; 

 and so the message carried to your brain, instead of consisting equally of the im- 

 pressions of red, green and violet, now consists chiefly of the last. The violet- 

 carrying nerves are unusually active after their short period of repose, while those 

 burdened with the red and green are fatigued by exposure, and scarcely act at all. 

 The resulting sensation is of an excess of violet light, mixed with green and red to 

 just the extent necessary to produce the subtle and very delightful blue you have just 

 seen. The effect vanishes as the tired nerves regain their normal sensitiveness, and 

 the white screen regains its ordinary appearance. 



After this brief discussion of the causes to which color is due, we are ready to 

 consider some of the theories of the color of the sky which have prevailed in more 

 recent times. 



Newton's hypothesis, that sky-blue is produced by inference, has been abandoned 

 long since, on account of difficulties which we cannot touch upon here, and has been 

 supplanted by the simpler theory that the color is due to atmospheric absorption 



' Helmholtz: Handbuch der physiologischen Optik, p. 317. 

 2 Maxwell: Philosophical Transactions, vol. CL, p. 78. 



Fig 1. 



