MEMORY. 645 



afterwards determine transition and action, its content 

 could not be conceived as one of the mind's permanent 

 meanings : tliat is all I mean by saying that its intellectual 

 value lies in after-memory. 



As a rule sensations outlast for some little time the ob- 

 jective stimulus which occasioned them. This phenomenon 

 is the ground of those ' after-images ' which are familiar in 

 the physiology of the sense-organs. If we open our eyes 

 instantaneously upon a scene, and then shroud them in 

 complete darkness, it will be as if we saAV the scene in ghostly 

 light through the dark screen. We can read off details in 

 it which were unnoticed whilst the eyes were open.* 



In svery sphere of sense, an intermittent stimulus, often 

 enough repeated, produces a continuous sensation. This 

 is because the after-image of the impression just gone by 

 blends with the new impression coming in. The effects of 

 stimuli may thus be superposed upon each other many 

 stages deep, the total result in consciousness being an in- 

 crease in the feeling's intensity, and in all probability, as 

 we saw in the last chapter, an elementary sense of the lapse 

 of time (see p. 635). 



coexisting with the multitude of other sensations which are juxtaposed in 

 consciousness alongside of this one persistent emotion which Is felt always 

 in the present tense. A lon,<r time is needed ere we can attain to forgetting 

 it, ere we can make it enter into the past. Hcei-et lateri letalis arundo. " 

 (Ibid 583) 



* This is the primary positive after-image. According to Helmlioltz, 

 one third of a second is the most favorable length of exposure to the light 

 for producing it. Longer exposure, complicated hy subsequent admission 

 of light to the eye, results in the ordinary negative and complementary 

 after-images, with their changes, which may (if the original impression 

 was bislliant and the fixation long) last for many minutes. Fechner gives 

 the name of memory-after images (Psychophysik, ir 492) to the instan- 

 taneous positive effects, and distinguishes them from ordinary after images 

 by the following characters : 1) Their originals must have been attended 

 to, only such parts of a compound original as have been attended to ap- 

 pearing. This is not the case in common visual after images. 2) The 

 elrain of attention towards them is inward, as in ordinary remembering, 

 not outward, as in observing a common after-image. 3) A short fixation 

 of the original is better for the memory-after-image, a long one for the 

 ordinary after-image. 4) The colors of the memory-after-image are 

 never complementary of those of the original. 



