358 PHYSIOLOGY CHAP. 



That the different spectral colours require different periods to 

 produce maximal excitation in the retina is shown by the fact 

 that the spectrum appears colourless and shortened at the red 

 end, when presented to the eye for a very brief period. If the 

 exposure is slightly prolonged it appears to consist only of a red 

 and a blue portion. According to Kunkel's delicate researches, 

 the red rays exert their maximal effect most rapidly, next the 

 blue, and more slowly still the green rays. 



The relatively slow course of retinal excitation explains a 

 whole series of facts of common observation. The phases of 

 waxing and waning of sensation explain why on glancing quickly 

 round, or on gazing steadily at some one who is running, we are 

 unable to distinguish the minute features of the objects or the 

 several phases of the movement, and have only a confused and 

 general impression. 



After-effect explains why the rapid movement of a red-hot 

 poker gives an impression of a fiery streak. By means of 

 instantaneous photography it is possible to reproduce images 

 perfect in every detail of the movements of one or more 

 individuals as they come into the range of the camera. If these 

 images are projected again, at the same speed at which they were 

 photographed, a more or less complete reproduction of the scene 

 is obtained, because the rapid succession of images fuses into a 

 y continuous, changing visual sensation. The Kinematographs, 

 which have become so popular of late, are only an applied form of 

 the physiological laws of the course of visual sensation more 

 exactly, of the after-effect of retinal excitation, and the resulting 

 positive after-images. 



These images are the more persistent, sharp, and intense in 

 proportion to the strength of the luminous stimulation, and 

 independently of its duration (Helmholtz). To obtain the 

 maximal effect, the duration of the stimulus should not exceed 

 -}{ sec. Persistent after - images follow exposure to a strong 

 electric spark, the duration of which is infinitesimal. An after- 

 image of the sun may last for some minutes, while that of a 

 moderately illuminated object disappears after 2 sees. (Aubert). 



- Charpentier, by rotating discs, succeeded in determining the 

 duration of the positive after-image under various conditions of 

 illumination and duration of the luminous stimulus. 



His results may be expressed as follows : with uniform 

 duration of the light-stimulus the persistence of the positive 

 image varies inversely with the illumination, approximately in 

 inverse ratio to its square root. With uniform illumination, the 

 persistence of the positive after-image varies inversely to the 

 duration of stimulus, approximately in inverse ratio to its square 

 root. So that increase in duration of stimulus acts, within 

 experimental limits, as increase of illumination. 



