RECURRENT VISION 89 



I have hitherto followed McDougall's excellent paper. It will be 

 readily appreciated that since the response differs in the number and 

 character of the pulses of sensation according to the intensity and nature 

 of the light stimulus, and since the stimulus acts upon two mechanisms 

 of different intensity-response and different latent period, the nature 

 of the phases of the response vary greatly according to the conditions 

 of the experiment. 



Fig. .37. Appearance of a narrow slit, 2° to 5^ in width, in a large disc, rotating at one 

 revolution per 3", before a milk-glass illuminated with one acetylene-gas burner, as 

 seen by the light-adapted eye which has been kept covered for about three minutes 

 and then fixed upon any point in or near the path of the moving slit — " Bidwell's 

 ghost." (McDougall.) 



It is not surprising, then, that under appropriate conditions v. Kries 

 found the following series of events : (1) a primary image ; (2) a short 

 dark interval ; (3) a secondary image, complementary in colour to 

 the primary (the satellite or ghost) ; (4) a second dark interval ; (5) a 

 tertiary image (Snellen, Bosscha), faintly tinged with the colour of 

 the primary ; (6) a third dark interval. The secondary image in this 

 series follows the primary by | — | sec. It is absent with foveal fixation. 

 It increases in brightness and extent in the early stages of dark adapta- 

 tion, but is absent after prolonged dark adaptation. It is also absent 

 with red light and its brightness corresponds to the scotopic value of 

 the exciting light (v. Kries). Much discussion has arisen around the 

 colour of the secondary image. According to Charpentier it is violet 

 with low, and colourless with high intensity. Hess describes it as 

 faintly tinged with the same colour as the primary i. With regard to 



1 Cf. v. Kries, Ztsch. /. Pmjchol u. Physiol, d. Sinnesorg. xii. 81, ISflO : Hess, Arch f. 



