RECURRENT VISION 



87 



The Primary Image. With momentary stimulation by a stationary 

 light McDougall found that a brief stimulus of low intensity provokes 

 a single brief pulse of sensation, and that a stimulus of higher intensity 

 provokes two, three, four or more pulses according to its intensity, 

 and that the extrafoveal region, being more sensitive under dark adapt- 

 ation than the fovea responds to a stimulus of any given low intensity 

 with a rather longer series of pulses than is provoked by the same 

 stimulus when applied to the fovea. If the stimulus is given by a radial 

 slit in a rotating disc the image is spread out into a sector, varying in 

 width according to the rapidity of rotation. This is due to the per- 

 sistency of the sensation beyond the time of stimulation. The sector is 

 not of uniform brightness, but is marked with alternate light and dark 

 bands which vary in intensity and width according to speed of rotation 

 and so on. These fan-like bands are commonly known as " Charpentier's 

 bands 1 ," though they had been previously described by Bidwell (1894). 



c, b a 



Fig. 36. Diagram illustrating the primary response to a single momentary stimulus. 

 The series of pulses of sensation commences at a and lasts through a period of about 

 f sec., represented by a i. The height to which each curve rises represents the 

 intensity of the pulse of sensation. The effects of a momentary stimulus of less 

 intensity are represented by a part of the series of curves, e.g., e ?. The overlapping 

 of the curves represents the fact that the intervals between the pulses of sensation 

 are not quite dark. (McDougall.) 



The Secondary Image. The above is not the only response. In the 

 case of momentary stimulation by a stationary light a secondary image 

 of much less luminosity seems to appear at a great distance at the 

 moment in which the pulsating image comes to an end, and rushing up 

 towards the eye to stop suddenly, filling the position of the primary 



1 Arch, de Physiol. iv. 541, 1892; vm. 677, 1890 



