THE SENSES. 553 



If the eye be fixed for a short time upon a white spot in a black 

 ground, and then suddenly directed toward a blank wall of white 

 or light gray color, a dark spot will appear upon it, of the same size 

 and figure with the white one previously observed. This is the " nega- 

 tive image " of the retinal impression. That part of the retina which 

 was first impressed by the rays from the white spot becomes less sensi- 

 tive ; and another white surface, looked at immediately afterward, 

 appears dark. On the other hand, those parts which were exposed 

 only to the dark ground, that is, to the comparative absence of light, 

 are more sensitive than before ; and the surface of the white wall, out- 

 side the central spot, consequently appears brighter. If a piece of 

 dark furniture against a white or gray wall be looked at steadily for a 

 short time, on shifting the eyes to a different part of the wall, the figure 

 of the chair or table will appear, with all its details of outline, expressed 

 in a lighter tint than that of the surrounding parts. 



Negative images may be produced in a still more simple manner. 

 Let a black ruler, about one inch wide, be laid upon a sheet of white 

 paper, and looked at steadily for thirty or forty seconds. If the ruler 

 be now suddenly removed, the eye remaining fixed, its image will 

 appear as a bright band upon the paper, gradually fading as the retinal 

 sensibility becomes equalized. 



The sensibility of the retina may be separately increased or dimin- 

 ished for different colors. If a black ruler be laid upon a blue cloth, 

 on taking it away a band appears in its place of a more intense blue 

 than the rest ; and if placed upon a red cloth, its negative image is 

 of a remarkably pure red, the remainder appearing of a dull brown. 

 But parts of the retina which have been fatigued by the continued 

 impression of one color are more sensitive to rays of the complemen- 

 tary hue ; since the latter have been for a certain time excluded. A 

 strip of red paper, placed on a white ground and suddenly removed, 

 leaves an image which is bluish-green ; and a green one leaves an image 

 with a tinge of red. The light from the white ground really contains 

 all the colors ; but an eye which has become less sensitive to green rays 

 will receive an impression in which the red predominates, and vice 

 versa. 



Owing to the variable sensibility of the retina, according to exposure, 

 an object, under some conditions, is most easily perceived by indirect 

 vision. It often happens that a small and feeble star may be momen- 

 tarily perceived by looking, not directly at it, but at some point in its 

 immediate neighborhood. The star is not seen distinctly under these 

 circumstances, because it is out of the line of direct vision. But its 

 light falls upon a part of the retina near the fovea centralis, where the 

 sensibility is more acute than usual, owing to its previous exposure 

 only to the dark sky ; while the fovea itself, which has been receiving 

 in succession the images of various stars, is comparatively deficient in 

 sensibility. When the visual axis is turned directly upon the faint 



