790 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



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." 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 243. 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 

 FIG. 243,-To illustrate the oscillatory interval of about 0.01 5". It thus appears that 



activity of the retina (Charpentier). ,1.1. ,-, , i 



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- 

 minated white surface (e. g. a sheet of white paper), we are usually unconscious 

 of any diminution in the intensity of the sensation, but it can be shown that 

 the longer we look at the paper the less brilliant it appears, or, in other words, 

 that the retina really becomes fatigued. To do this it is only necessary to place 

 a disk of black paper on the white surface and to keep the eyes steadily fixed 

 for about half a minute upon the centre of the disk. Upon removing the disk 

 without changing the direction of the eyes a round spot will be seen on the 

 white paper in the place previously occupied by the disk. On this spot the 

 whiteness of the paper will appear much more intense than on the neighboring 

 portion of the sheet, because we are able in this experiment to bring into direct 

 contrast the sensations produced by a given amount of light upon a fresh and 

 a fatigued portion of the retina. 2 



1 Charpentier : Archives de Physiologic, 1892, pp. 541, 629 ; and 1896, p. 677. 



2 Although the retina is here spoken of as the portion of the visual apparatus subject to 

 fatigue, it should be borne in mind that we cannot, in the present state of our knowledge, dis- 

 criminate between retinal fatigue and exhaustion of the visual nerve-centres. 



