776 SIGHT. 



part of the retina on which the image of the patch fell becomes tired ; hence, 

 the white light, coming from the white ground subsequently looked at, which 

 falls on this part of the retina, does not produce so much sensation as in 

 other parts of the retina; and the image, consequently, appears gray. And 

 so, in the other instance, the whole of the retina is tired, except at the patch ; 

 here the retina is for a while most sensitive, and hence the white negative 

 image. 



When a red patch is looked at, the negative image is a green-blue, that 

 is, the color of the negative image is complementary to that of the object. 

 Thus, also, orange produces a blue, green a pink, yellow an indigo-blue, neg- 

 ative image, and so on. This, too, can be explained as a result of exhaus- 

 tion on either hypothesis of color vision. When the colored patch is looked 

 at, one of the three primary color sensations is much exhausted, and the 

 other two less so, in varying proportions, according to the exact nature of 

 the color of the patch ; and the less exhausted sensations become prominent 

 in the after-image. Thus, the red patch exhausts the red sensation, and the 

 negative image is made up chiefly of green and blue sensations, that is, 

 appears to be greenish-blue, or bluish-green, according to the tint of the red. 

 On the other hypothesis, we may suppose that, owing to the continued effect 

 of looking at the red patch, dissimilation of the red-green substance 

 becomes less and less, leading to a prominence and, indeed, to an actual in- 

 crease of the process of assimilation of the same substance ; hence, the sensa- 

 tion of green dominating in the negative image. 



Similarly, when the eye, after looking at a colored patch, is turned to a 

 colored ground, the effects may easily be explained by reference to the com- 

 parative exhaustion of the color sensations excited by the patch and the 

 ground respectively ; if a yellow ground be chosen after looking at a green 

 object, the negative image will appear of a reddish-yellow, and so on. 



670. The theory of three primary sensations does not so readily ex- 

 plain why negative images should make their appearance without any sub- 

 sequent stimulation of the retina. When the eyes are shut and all access 

 of light, even through the eyelids, carefully avoided, the field of vision is not 

 absolutely dark ; there is still a sensation of light, the so-called " proper 

 light " of the retina. If a white patch on a black ground be looked at for 

 some time, and the eyes then shut, a negative (black) image of the spot 

 will be seen on the ground of the " proper light " of the retina, having in 

 its immediate neighborhood a specially bright corona. So, also, if a window 

 be looked at and the eyes then closed, the positive after-image with bright 

 panes and dark sashes gives rise to a negative after-image with bright sashes 

 and dark panes ; and similar effects appear with colors. These and similar 

 facts have been largely used in support of the Hering theory. When the 

 eye has been looking at red, and so has caused dissimilation of the red- 

 green substance, mere rest, as on shutting the eyes, favors assimilation of the 

 same substance and thus leads to a sensation of green. And the rhythmic 

 oscillations from one color to its correlative and back again, frequently 

 observed under these conditions and which point to assimilation and dissimi- 

 lation alternately gaining the upper hand, are not without analogies in other 

 common instances of protoplasmic metabolism. 



VISUAL PERCEPTIONS. 



671. Hitherto we have studied sensations only, and have considered 

 an external object, such as a tree, as simply a source of so many distinct 

 sensations, differing from each other in intensity and kind (color). In the 

 mind these sensations are coordinated into a perception. We are not only 



