SENSE OF SIGHT. 647 



white spot in the middle of a black ground, and then suddenly directed 

 toward a blank wall of a uniform white or light gray color, a dark spot 

 will appear at its centre, of the same apparent size and figure with the 

 white one previously observed. This is the " negative image" of the 

 retinal impression. That part of the retina which was first impressed 

 by the rays from the white spot becomes less sensitive to light ; and 

 another white surface, looked at immediately afterward, appears darker 

 than usual. 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, outside 

 the central spot, appears brighter than usual. It is not necessary that 

 the contrast in hue between the different parts of a retinal image should 

 be as strong as that of black and white, in order to produce this effect. 

 Any decided difference in illumination will be sufficient. It is not even 

 essential to look at a different background, to observe the appearance 

 in question. If a piece of furniture of dark wood be placed against a 

 blank wall of white or gray surface, and looked at steadily for a short 

 time, on shifting the eyes to a different part of the same 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. 



The above effect may be also produced in a still more simple man- 

 ner. 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 removed by a sudden motion, the eye remaining fixed, its 

 image will appear as a bright band upon the paper, fading gradually as 

 the sensibilitjr of the retina becomes equalized in its different parts. 



If the figure which is thus examined be a colored one, its negative 

 image, subsequently produced, will present a complementary hue to that 

 of the original object. A strip of red paper placed upon the white 

 sheet, and then suddenly removed, leaves a negative image which is 

 bluish-green ; and a green one leaves an image which has a decided 

 tinge of red. This shows that the sensibility of the retina may be in- 

 creased or diminished separately for the different colored rays of the 

 luminous beam. While looking at a red object, the retina becomes less 

 sensitive to the red rays, but more so for those at the opposite end of 

 the spectrum, and vice versa ; so that, on looking subsequently at a 

 white object, the negative image exhibits a tint corresponding to the 

 rays for which the retina has remained most sensitive. That this is the 

 mechanism of the production of complementary colors in negative 

 images becomes evident on simplifying the experiment. If the black 

 ruler be laid upon a book bound in blue cloth, on taking it away the 

 band which remains in its place is of a more intense blue than the rest. 

 If a red book be used for the same purpose, the negative image of the 

 ruler presents a remarkably pure red color, while the remainder of the 

 surface appears of a dull brown. 



The variable sensibility of the retina, according to its exposure, 

 affords an explanation of the well-known fact, that under some condi- 



