344 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



vary with the rate of rotation of the disk and with the amount of exhaustion 

 of the retina. Chromatic effects due to a similar cause are also to be seen 

 when a disk, such as is shown in Figure 154, B (known as Benham's spectrum 



A B 



Fig. 154. — Disks to illustrate the varying rate at which colors rise to their maximum of sensation. 



top), is rotated with moderate rapidity. The concentric bands of color appear 

 in reverse order when the direction of rotation is reversed. The apparent 

 movement of colored figures on a background of a different color when the 

 eye moves rapidly over the object or the object is moved rapidly before the 

 eye seems to depend upon this same retinal peculiarity. The phenomenon 

 may be best observed when small pieces of bright-red paper are fastened upon 

 a bright-blue sheet and the sheet gently shaken before the eyes. The red 

 figures will appear to move upon the blue background. The effect may be 

 best observed in a dimly-lighted room. 



In this connection should be mentioned the phenomenon of " recurrent 

 images " or " oscillatory activity of the retina." x This may be best observed 

 when a black disk containing a white sector is rotated at a rate of about one 

 revolution in two seconds. If the disk is brightly illuminated, as by sunlight, 



and the eye fixed steadily upon the axis of rota- 

 tion, the moving white sector seems to have a 

 shadow upon it a short distance behind its ad- 

 vancing border, and this shadow may be followed 

 by a second fainter, and even by a third still 

 fainter shadow, as shown in Figure 155. The 

 distance of the shadows from each other and 

 from the edge of the sector increases with the rate 

 of rotation of the disk and corresponds to a time 

 ro illustrate the oscillatory interval of about 0.015". It thus appears that 



activity of the retina (Charpentier). . 



when light is suddenly thrown upon the retina 

 the sensation does not at once rise to its maximum, but reaches this point by 

 a sort of vibratory movement. The apparent duplication of a single very 

 brief retinal stimulation, as that caused by a flash of lightning, may perhaps 

 be a phenomenon of the same sort. 



Fatigue of Retina. — When the eye rests steadily upon a uniformly illu- 

 1 Charpentier: Archives de Physiofogie, 1892, pp. 541, ''>•_!'.) ; and 1886, p. 677. 



