678 COMPARATIVE ELECTRO-PHYSIOLOGY 



for a longer time than that of feeble, may be exhibited in. an 

 interesting manner as follows. A simple design is made 

 with magnesium powder and fired in a dark room. A 

 second observer, unacquainted with the design, observes the 

 flash and closes his eyes. The instantaneous flash, 

 obscured as it is by dense smoke, does not at once produce 

 any definite impression. In the retina, however, the ob- 

 scuring image of the smoke, being of little luminosity, 

 quickly passes off, and the after-effect of the brilliant flash, 

 thus separated from the obscuring smoke, grows into perfect 

 distinctness. In this manner I have often been able, by sub- 

 sequent closure of the eyes, distinctly to observe luminous 

 phenomena of brief duration, which, while the eyes were 

 open, had been indistinguishable. 



We have thus touched upon the question of the time 

 required for the obliteration of a mental impression. 

 Another interesting aspect of this subject lies in the rate at 

 which fading takes place, or, in other words, the rate of 

 molecular recovery. In the curve of recovery we find that 

 this rate is at first very rapid, and becomes increasingly slow 

 with the descent of the curve. This is also the characteristic 

 of the process of forgetting, Ebbinghaus, for instance, found 

 that the forgetting of a series of ' nonsense syllables/ at first 

 quick, became increasingly slower with time. 



Certain excitable tissues, such as nerve and cardiac 

 muscle, give responses to strong stimulus, which, as we have 

 seen, are multiple in character. The retina, again, under 

 intense stimulus of light, exhibits multiple after-excitations, 

 which may be detected by a galvanometer (p. 426). This 

 fact explains the multiple after-image often seen on closing 

 the eyes after strong light. Another proof that these 

 multiple after-images are physiological lies in the fact that 

 their periodicity is modified by a previous condition of rest 

 or activity. Thus, early in the morning, when fresh from 

 rest, this period I find to be at its shortest, and later in the 

 day to become gradually longer, owing to growing fatigue. 

 In a given instance, the period at 8 A.M. was 3 seconds, 



