Such an illustration shows how trivial an impression may be thus registered and preserved.  But, if, on such an inorganic surface, an impression may thus be indelibly marked, how much more likely in the purposely- constructed ganglion!  A shadow never falls upon a wall without leaving thereupon a permanent trace, a trace which might be made visible by resorting to proper processes.  Photographic operations are cases in point.  The portraits of our friends, or landscape views, may be hidden on the sensitive. surface from the eye, but they are ready to make their appearance as soon as proper developers are resorted to.  A spectre is concealed on a silver or glassy surface until, by our necromancy, we make it come forth into the visible world.  Upon the walls of our most private apartments, where we think the eye of intrusion is altogether shut out and our retirement can never be profaned, there exist the vestiges of all our acts, silhouettes of whatever we have done.

If, after the eyelids have been closed for some time, 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 indefinite darkness beyond us.  We may satisfy ourselves that this is not a fiction, 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 the 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 eyeball, it suddenly vanishes.

Such a duration of impressions on the retina proves that the effect of external influences on nerve-vesicles is not necessarily transitory.  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, as artists say, months subsequently in New York—­the images coming out, after the long voyage, in all their proper forms and in all their proper contrast of light and shade.  The photograph had forgotten nothing.  It had equally preserved the contour of the everlasting 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?  Is this the explanation 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 hung micrographs of the living and the dead, of scenes that we have visited, of incidents in which we have borne a part?  Are these abiding impressions mere signal-marks, like the letters of a book, which impart ideas to the mind? or are they actual picture-images, inconceivably smaller than those made for us by artists, in which, by the aid of a microscope, we can see, in a space not bigger than a pinhole, a whole family group at a glance?