648 PSYCHOLOGY. 



£rst condition which makes a thing susceptible of recall 

 after it has been forgotten is that the original impression 

 of it should have been prolonged enough to give rise to a 

 recurrent image of it, as distinguished from one of those pri- 

 mary after-images which very fleeting impressions may 

 leave behind, and which contain in themselves no guarantee 

 that they will ever come back after having once faded away,* 

 A certain length of stimulation seems demanded by the 

 inertia' of the nerve-substance. Exposed to a shorter in- 

 fluence, its modification fails to 'set,' and it retains no 

 effective tendency to fall again into the same form of vibra- 

 tion at which the original feeling was due. This, as I 

 said at the outset, may be the reason why only ' substantive ' 

 and not 'transitive' states of mind are as a rule recol- 

 lected, at least as independent things. The transitive states 

 pass by too quickly. 



ANALYSIS OF THE PHENOMENON OF MEMORY. 



Memory proper, or secondary memory as it might be 

 styled, is the knowledge of a former state of mind after it 

 has already once dropped from consciousness ; or rather it 

 is the knoicledge of an event, or fact, of which meantime we 

 have not been thinking, imth the additional consciousness that 

 we have thought or experienced it before. 



* The primary after-image itself caunot be utilized if the stimulus is too 

 brief. Mr. Cattell found (Psychologische Studien, in. p. 93 ff.) that the 

 color of a light must fall upon the eye for a period varying from 0.00275 

 to 0.006 of a second, in order to be recognized for what it is. Letters 

 of the alphabet and familiar words require from 0.00075 to 0.00175 

 sec. — truly an interval extremely short. Some letters, E for example, are 

 harder than others. In 1871 Helmholtz and Baxt had a.scertaiued that 

 when an impression was immediately followed by another, the latter 

 quenched the former and prevented it from being known to later conscious- 

 ness. The first stimulus was letters of the alphabet, the second a bright 

 white disk. "With an interval of 0.0048 sec. between the two excita- 

 tions [I copy here the abstract in Ladd's Physiological Psychology, p. 480], 

 the disk appeared as scarcely a trace of a weak shimmer ; with an interval 

 of 0.0096 sec, letters appeared in the shimmer— one or two which could 

 be partially recognized when the interval increased to 0.0144 sec. When 

 the interval was made 0.0192 sec. the objects were a little more clearly 

 discerned ; at 0.00336 sec. four letters could be well recognized ; at 0432 

 «ec., five letters ; and at 0.0528 sec. all the letters could be read." (Pflugcr'a 

 Archiv. iv. 325 ff.) 



