MEMORY 679 



which had lengthened to 5 seconds by 3 P.M. ; and was 

 further prolonged to 6*5 seconds by 1 1 P.M. 



One way of exhibiting the after-images in the retina is, 

 as we have already seen, by means of a stereoscope con- 

 taining two incised slits inclined to each other, instead of 

 photographs. On looking through this at the bright sky for 

 ten seconds, or longer, a composite image is formed of an 

 inclined cross. The eyes are now closed, and the first effect 

 noticed is one of darkness, due to the molecular rebound. 

 By reason of the Binocular Alternation of Vision, already 

 referred to, one luminous arm of the inclined cross now 

 projects itself aslant the dark field, and then slowly disap- 

 pears, after which the second, perceived by the other eye, 

 shoots out suddenly in a direction athwart the first. This 

 multiple alternation proceeds for a long time, and produces 

 the curious effect of two luminous blades crossing and re- 

 crossing each other. At first the after-images of the cross, 

 seen with the eyes closed, are very distinct, so distinct that 

 any unevenness in the design at the edges of the slanting 

 cuts can be made out clearly. There is here no doubt of 

 the * objective ' nature of the strain impressed on the retina, 

 which, on the cessation of direct stimulus of light, gives rise 

 to after-oscillations with concomitant visual recurrences. 

 This recurrence may be taken as a proof of the existence of 

 physical strain in the retina. The recurrent after-image is 

 very distinct at the beginning, but becomes fainter with each 

 repetition. A time comes when it is difficult to tell whether 

 the image is the objective after-effect due to previous strain, 

 or merely an effect of ' memory.' There is, in fact, no hard- 

 and-fast line of demarcation between the two one merges 

 simply into the other. In connection with this, it is 

 interesting to note that some of the phenomena of memory 

 also are admitted to be recurrent. 



Visual impressions and their recurrence often persist 

 for a very long time. It usually happens J:hat, owing to 

 weariness, the recurrent images disappear, but in some in- 

 stances, long after this disappearance, they will spontaneously 



