SUCCESSIVE INDUCTION OR AFTER-IMAGES 103 



forward evidence to show that the after-effects of a primary stimulus 

 are much more prolonged than has been generally recognised, and these 

 later after-effects may materially alter the character of the after-image. 

 So complex indeed are the effects that the greatest care should be 

 exercised in making theoretical deductions from after-images. 



With primary stimuli of considerable intensity a very short exposure 

 is necessary to induce the after-image, which may be so strongly 

 developed as to nullify successive stimuli to the same part of the 

 retina for a considerable time ("positive scotoma"). W. Tschermak 

 stated that primary stimuli with coloured lights of such low intensity 

 as to be below the chromatic threshold were followed by coloured 

 after-images, but it was not found to be so by Titchener and Pyle 1 . 



Analogy with other physiological processes would lead us to the 

 conclusion that the positive after-image results from a persistence of 

 those processes which have been set in action by the primary stimulus, 

 and that the negative after-image is the expression of a diminished 

 excitability of the stimulated area to fresh stimuli (Fechner, v.Helmholtz). 

 The fact that a stimulus usually gives rise to a rhythmical response, as 

 has already been shown (p. 87), makes the periodical variations in the 

 after-image, which were first described by Plateau, intelligible. Under 

 suitable conditions, with the eyes motionless, the negative image 

 disappears and reappears at intervals of three or four seconds, sometimes 

 alternating with positive images. Sudden movements of the eyes, 

 distraction of the attention, and other like influences abolish the after- 

 image ; hence they are seldom noticed in every-day life. 



Moreover with prolonged or strong stimuli the condition of altered 

 sensibility of the affected part of the retina may persist materially 

 longer than the apparent after-effect as shown by the negative after- 

 image. When the latter seems to have quite disappeared if the eye is 

 thoroughly darkened the affected portion of retina gives rise to a sensa- 

 tion of blackness which is more intense than that derived from the 

 surrounding areas. 



It might seem that the effect of a single isolated stimulus, without 

 any secondary stimulus, was a simple matter. This is, however, 

 by no means the case. The negative after-image of a white object 

 appears as a black spot in the midst of a less black field. There is there- 

 fore a sensation of blackness which is blacker than that obtained from 

 the eye when all external light is excluded. The entopic sensation when 

 the eyes have long been completely excluded from light is variously 



1 Proc. Amer. Pli.ilos. Soc,. XL. 300. 1908. 



