TIME-RELATIONS OF RETINAL STIMULATION. 863 



the complementary color) and lasts of a second, being the more distinct the 

 shorter the light-stimulation. Then follows the positive after-image, whose 

 duration increases with the intensity of the illumination. 



"That the impression of any image in the eye persists for some time we know 

 as a physiological phenomenon; excessive duration of such an impression, how- 

 ever, may be considered pathological. The weaker the eye the longer does the 

 image remain in it. The retina does not recover so quickly, and the effect may 

 be looked upon as a sort of paralysis. This is not to be wondered at in the case 

 of dazzling images. If one looks directly at the sun, he may carry the image 

 about for several days. The same is also relatively true of images that are not 

 dazzling. Biisch relates of himself that an engraving remained before his eye, 

 completely with all its details, for seventeen minutes" (Goethe). 



Experiments and Apparatus for Demonstrating After-images. (i) The ap- 

 pearance of a ring of fire on rapid rotation of a coal. (2) The thaumatrope of 

 Paris: a pasteboard card contains, for example, on one side the picture of a torso- 

 statue, on the other side the picture of the remaining portions drawn in ap- 

 propriate positions. If the card be rotated so that the two surfaces in al- 

 ternation are rapidly turned toward the observer, the statue appears complete. 

 (3) The phanakistoscope or the stroboscopic discs. Objects are drawn on a 

 disc or cylinder in succession, so that the drawings represent successive details 

 of a continuous movement. On rapid rotation of the disc, the observer, looking 

 through a small opening, sees the moving images pass before the eye, each phase 

 rapidly replacing the preceding. As the impression of each image remains until the 

 following one takes its place, one and the same figure seems to go through the succes- 

 sive movements continuously. The apparatus, at present popularized by Anschutz 

 in the form of the zoetrope (perfected by Edison as the kinematograph or kineto- 

 scope) , was not discovered by the two investigators mentioned , in 1 83 2 , as is generally 

 supposed; but it was described by Cardanus as early as 1550. It may be used also 

 scientifically for the representation of certain movements: as, for example, of 

 spermatozoids and ciliated epithelial cells; likewise the movements of the heart 

 and of walking may be instructively shown and analyzed. (4) The color-top 

 contains in the sectors on its. surf ace the colors that are to be mixed. As the 

 color of each sector causes a stimulation of the retina, lasting throughout the 

 revolution of the top, all of the colors must be seen simultaneously, and be per- 

 ceived as a mixed color. 



Occasionally, especially if the retinal excitation is of considerable 

 duration and intensity, a negative after-image appears, instead of the posi- 

 tive. The former is characterized by the fact that bright portions of the 

 object appear dark, and the colored portions in corresponding contrast- 

 colors. 



Examples of Negative After-images. After gazing for a long time at a brightly 

 illuminated white window, and then closing the eyes, there results the impression 

 of a window with bright cross lines, and dark panes. Colored negative after- 

 images are shown beautifully by Norrenberg's apparatus: the eye is fixed for 

 some time on a colored surface, such as a yellow card, in the center of which 

 is pasted a small blue square. Suddenly a white screen is dropped in front of 

 the card, and the white surface then appears bluish, with a yellow square in 

 the center. 



The usual explanation of the dark negative after-images is that the retinal 

 elements are so fatigued by the light that they become less irritable for a time, so 

 that in these portions of the retina the light can be only faintly perceived, and 

 darkness, therefore, must prevail. Bering explains the dark after-images as 

 resulting from the process of assimilation of the black-white visual substance. 



For the explanation of colored after-images the Young-Helmholtz theory 

 assumes that by the action of the color, for example red, the retinal elements 

 for this particular color are paralyzed. If, now, the eye looks at a white surface, 

 this mixture of all colors appears as white minus red, that is green (the contrast- 

 color, which in bright daylight lies close to the complementary color). Accord- 

 ing to Hering, this contrast-colored after-image is the result of the assimilation 

 of the corresponding colored visual substance, in the case cited, of the "red-green." 

 From the beginning of a momentary illumination until the appearance of an 

 after-image 0.344 second elapses. 



