116 ANXUAL OF SCIENTIFIC DISCOVERY. 



made visible. In many cases we have ascertained what the appropriate 

 agent is ; our failure in others is due to the imperfection of our knowl- 

 edge, and not to any impossibility in the operation. Time seems to have 

 so little influence on these effects, that I can conceive it possible, if a new 

 vault should hereafter be opened in the midst of an Egvptian pyramid, 

 for us to conjure up the swarthy forms of the Pharaonic officials who 

 were its last visitors, though forty centuries may have elapsed since their 

 departure. 



But let us see how these facts bear, in a most important manner, in 

 the case of man. 



If after the eyelids have been closed for some time, as when we first 

 awake in the mornnig, we suddenly and steadfastly gaze at a brightly 

 illuminated object, and then quickly close the lids again, a phantom 

 image is perceived in the infinite darkness before us. We may satisfy 

 ourselves that this is not a fiction of the imagination, but a reality; for 

 many details that we had not time to examine in the momentary glance, 

 may be contemplated at our leisure in the phantom. We may thus 

 make out the pattern of such an object as a lace curtain hanging in the 

 window, or the branches of a tree bcvond. By degrees the image be- 



*/ * fj 



comes less and less distinct ; in a minute or two it has disappeared. It 

 seems to have a tendency to float awav in the vacancy before us. If 



* *' 



you attempt to follow it by moving the eyeball, it suddenly vanishes. 



"Now the condition that regulates the vanishing phantom-images 

 on the retina is, that when they have declined in vigor to less than 

 -gif of the intensity they had while in presence of the object that 

 formed them, they cease to disturb the sight. This principle is illus- 

 trated when a candle-flame is held opposite to the sun, or any light 

 having more than ('14 times its own brilliancy. It then ceases to be vis- 

 ible. The most exact of all known methods for measuring light that 

 by the extinction of shadows is an application of the same principle. 



"But the great fact that concerns us is this : Such a duration of 

 impressions on the retina of the eye demonstrates that the effect of ex- 

 ternal influences on nerve vesicles is not necessarily transitory. It 

 may continue for a long time. In this there is a correspondence to the 

 duration, the emergence, the extinction of impressions on photographic 

 preparations. Thus I have seen landscapes and architectural views 

 taken in Mexico, 'developed 1 as artists say months subsequently ; 

 the images coming out, after the long voyage, in all their proper forms 

 and in all their contrast of light and shade. The photograph had for- 

 gotten nothing. It had equally preserved the contour of the everlast- 

 ing mountains and the passing smoke of a bandit fire. 



" Are there then contained in the brain more permanently, as in the 

 retina more transiently, the vestiges of impressions that have been 

 gathered by the sensory organs? Do these constitute the basis of 

 memory the mind contemplating such pictures of past things and 

 events as have been committed to her custody. In her silent galleries 

 are there' Lung micrographs of the living and the dead, of scenes that 

 we have visited, of incidents in which we have borne apart? Are 

 these abiding imiMvssioi-s mere .signal-marks, like the letters of a book, 

 which impart ideas to the mind, or are they actual picture-images, in- 

 conceivably smaller than tl:o-ic made for us by artists, in which, by the 

 aid of a microscope, we can see, in a space not bigger than a pin-hole 

 a whole family group at a glance? 



