68 WALTER S. HUNTER 



subsidiary phenomena serving as a means for the effective func- 

 tioning of the after-image processes. But after-images can 

 hardly persist long enough to account for the maximum periods 

 of delay attained by this method of solution. Such delays 

 varied from lo sees, for the rats to 25 sees, for the raccoons 

 and 5 mins. for one dog. All of the conditions of the test were 

 distinctly unfavorable for any persistence of possible after- 

 images. The light was weak in intensity (3 c.p.). Its average 

 duration of exposure was approximately but 5 sees. Any ap- 

 proximation to steady fixation either during or subsequent to 

 the exposure of the light was the exception. The animals as a 

 rule were continually on the move, nosing and clawing at the 

 face of the release box both during the exposure and the period 

 of delay. Steadiness of fixation after the perception of the 

 stimulus is a very essential condition for the development of 

 after-images. Movements of the head or eyes tends not only to 

 prevent their appearance but also to destroy them when present. 

 The significance of these conditions is more apparent by re- 

 calling the fact that any extended duration of after-images is 

 an exceedingly rare phenomenon in the normal perceptual activ- 

 ities of humans. Fixation is too short and changeable for their 

 development. Since we are forced to argue from analogy with 

 human conditions, one must also distinguish between the pos- 

 sible presence of such processes and the ability to perceive 

 them. The mind tends in the interest of clear vision to over- 

 look and neglect such processes as it does in the case of entoptic 

 phenomena. With many people the ability to see after-images 

 involves a previous knowledge of their existence and some 

 degree of training and practice in their observation. In other 

 words, after-images as persistent objects of consciousness are 

 a product of the laboratory, and the assumption of their effec- 

 tive existence as guides to conduct in the normal perceptual 

 activity of an animal is exceedingly questionable. After-images 

 exhibit the phenomenon of intermittence. No high percentage 

 of correctness of response could result from such a cue, as its 

 presence at the moment of response would be a chance coinci- 

 dence. The theory of after-images, moreover, is entirely un- 

 necessary, as we have the possibility that these motor attitudes 

 of orientation may themselves serve as a sufficient guide to 



