PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 633 



yond these, sensori-motor acts. In dreaming, ideas are crowded. Dreara- 

 ing-, or a combination, must be distinguished by purely somnambulistic 

 acts. Perhaps in the common form of somnambulism, dreaming is, to a 

 greater or less extent, associated with it. Thus somnambulism is pre- 

 sented under various foi-ms, according to the absolute and relative degree 

 of activity of the different senses, and the condition of the central lobes. 

 All forms occur, from merely turning in bed to walking, talking, writing, 

 &c. And as in somnambulism some degree of activity by the cerebral lobe 

 may be associated with an active state of the sensorium, so in dreaming 

 some degree of activity in the sensorium may be combined with an active 

 state of the cerebral lobes. In the so called state of somnambulism, in 

 which acts are performed which involve a considerable exercise of the 

 mental powers, the simply somnambulistic state must be combined with 

 vivid dreaming. In this combination, so many of the faculties are more or 

 less active, so few, if any, are completely at rest, that the individual is 

 more awake than asleep. 



Photological. 



The distinguished Dr. Draper of the New York University, in a late dis- 

 course, thus speaks of the impressions made upon us by light : "If after 

 the eyelids have been closed for sometime, as when we first awake in the 

 morning, 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 identify 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 branches of a tree 

 beyond. By degrees the image becomes less and less distinct ; in a minute 

 or two it has disappeared. It seems to have a tendency to float away in 

 the vacancy before us. If we attempt to follow it by moving the eye-ball, 

 it suddenly vanishes. 



" Now the condition that regulates the vanishing phantom of images on 

 the retina is, that when they have declined in vigor to less than l-64th of 

 the intensity they have while in presence of the object that formed them, 

 they cease to disturb the sight. This principle is illustrated when a candle- 

 flame is held opposite to the sun, or any light having more than sixty-four 

 times its own brilliancy. It then ceases to be visible. 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 impres- 

 sions on the retina of the eye demonstrates that the effect of external influ- 

 ences on nerve vesicles is not necessarily transitory. It may continue for 

 a long time. In this there is a correspondence to the duration, the emer- 

 gence, the extinction ef impressions on the photographic preparation. 

 Thus I have seen landscapes and architectural views taken in Mexico, 

 developed — 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 forgotten nothing. It had equally 



