1876.] ^^ [Hartshorne, 



of the same color might affect the sight if they were alone, they are, as it 

 were, overpowered by the unimpeded complementary color-rays. This 

 consideration appears to me to meet an objection to my "interference" 

 theory, suggested by Prof. Houston; derived from the necessary relation of 

 wave-length of rays for interference. If only a moiety of the ether-waves of 

 a certain color in a beam of light afford the required opposition of phase in 

 undulation, their an'est or subtraction may yet explain the relative lessening 

 of the intensity of that color, so as to cause it to cease to be perceived; the 

 complementary color-rays having twice as great a strength. 



When color shadows are produced by intercepting transmitted colored 

 light, the shadows must fall on a white ground, or they do not liave the 

 color complementary to that which is transmitted. Here, again, the eyes 

 becoming saturated with a certain color, its specially responsive nerve- 

 elements in each retina are set in vibration; and, as in the other case, inter- 

 ference occurs between these vibrations and the ether-undulations corres- 

 ponding with the same color, in the reflected (diff"used) white light Let 

 the shadow of the intercepting object fall upon a black surface, and the 

 complementary color disappears. Nor will it be seen when only monochro- 

 matic light is present in the apartment. 



Again, the above-described over-tints or flushes, and complementary 

 interrupting spots, strips or bands, upon a colored ground seen through 

 thin white paper, depend upon the lohite light reflected by that paper, for 

 their complementary color. If the paper be removed, the actual diiference 

 between the ground and the spots, strips or bands laid upon it, will be 

 clearly seen . Replace it, and at once a strong contrast re-appears ; as before, 

 I hold, resulting from the interference of equivalent vibrations. 



Lastly, when a very strong impression is made upon the eyes, as by look- 

 ing at the sun through a colored glass, the excitement of the nerve-elements 

 is so great as to persist in the same manner; producing continued sensation of 

 the same color, when the eyes are closed, for some time. But, when the eyes, 

 in this state, are opened upon a white ground, again the waves of the same 

 color-rays in the white light kill, as it were, that color in the eyes; or, in 

 other words, arrest, for the time, enough of the existing retinal vibrations 

 to annul tlieir effect in sensation, so that only the remaining (complemen- 

 tary) rays of light are perceived. Bright reflected white light thrown 

 even upon the closed eyes will have this effect. 



An analogous explanation will suffice for the secondary or derived 

 spectra, mentioned on a previous page. Looking steadily at a small 

 white spot on a red ground, for example, the white spot becomes green; 

 turning thence to a white ground, a large green spectrum is seen, with a 

 red spot in the centre. The latter I call a secondary spectrum. While 

 looking at the red ground, vibration of the red-resonating retinal nerve- 

 elements began, and was extended, by irradiation, to those covering, in 

 vision, the white spot in the centre; being, there, sufficient to neutralize the 

 red rays of the white, and thus allowing only green to be seen, by vibra- 

 tions of the green-resonating rods and cones of that part of the retina. 



