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.