9')9 



Hartshorne.; ^— j [April 21. 



ful to refer to some facts fivmiliar to most of those who have given attention 

 to physiological optics. 



When the eyes liave been directed for a short time to a bright object, as 

 a piece of polished silver, and they are then turned towards a white wall 

 or other light surface, a dark spot or figure, having the outline of the bright 

 object first seen, is beheld upon the white or light surface. Or if, on the 

 contrary, a dark body, as an inkstand, for instance, be looked upon steadily 

 for awhile, on turning the eyes toward a light ground, a brilliant whit« 

 corresponding figure is seen. These are called negative spectra. If, now, 

 instead, we look fixedly at a colored object, and afterwards turn the eyes 

 to a white ground, or, if the colored body be upon a white surface&nii, after a 

 time, this body be removed, we we will see, in either case, a spectrum hav- 

 ing the color complementary to that of the object beheld. These are, also, 

 commonly called • negative spectra; the expression complementary color- 

 spectra will designate them more accurately, without regard to theory. 



It has been noticed that, near sunset, the rays of the sun passing through 

 an orange-colored cloud cast bluish shadows; and likewise, the shadows of 

 of objects seen behind red curtains are apt to be green. If the light either 

 of the sun or of a strong artificial light be made to pass through a pane of 

 colored glass, so as to fall upon a white ground, and a slender object, as a 

 rod, or the hand, intercepts the colored light, the shadow thrown has the 

 complementary color to that of the transmitting pane. Red glass will thus 

 throw green shadows, green glass red, orange glass blue, &c. If we look 

 at the shadow, so throAvn, through a tube, so that it alone is seen, it is per 

 ceived as a shadow without color. Also, if the same shadow falls upon a 

 black surface, no shadow appears. 



The above mentioned facts have all been repeatedly observed; and but 

 one explanation, so far as I can ascertain, has, as yet, been projiosed for 

 them.* It is that suggested by Dr. Thomas Young, and accepted by Helm- 

 holtz. Young's theory of colors, being the application to vision of Johannes 

 Miiller's general theory of special sensation (as depending upon the charac- 

 ter of the sensory organ more than on that of the external cause), asserted 

 the existence of difl'erent susceptibilities to color rays in different portions of 

 the retina; or among the different optic nerve-filaments. With our present 

 knowledge of the minute structure and relative functions of these parts, 

 such special susccptilnlities, viz: to red, green and violet light, as Young 

 designated the primary colors, or to red, yellow and blue, as thej' are more 

 commonly named, may be referred to the posterior layer of the retina; the 

 layer oi rods and cones of Jacob and Max Schultze. Helmholtz adopts this 

 theory of Young, as the only one giving any clue at all toward solving the 



*S1ih;o this was written, I have read, in Lardner's treatise on Optics, a brief 

 stateinont of Plateau's theory, of an '•oscillatory movement" of tlie whole 

 retina, when affected with litjht, in connection with what he designates as a 

 resislanee of the retina to the action of any color, and a tendency to resume its 

 ordinary condition, with a more or less intense force. Tiiere seems to be ex- 

 tremely little evidence in favor of this theory; and it has apparently fallen out 

 of view In later works. 



