uddenly 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?

The phantom images of the retina are not perceptible in the light of the
day. Those that exist in the sensorium in like manner do not attract our
attention so long as the sensory organs are in vigorous operation, and
occupied in bringing new impressions in. But, when those organs become
weary or dull, or when we experience hours of great anxiety, or are
in twilight reveries, or are asleep, the latent apparitions have their
vividness increased by the contrast, and obtrude themselves on the
mind. For the same reason they occupy us in the delirium of fevers, and
doubtless also in the solemn moments of death. During a third part of
our life, in sleep, we are withdrawn from external influences; hearing
and sight and the other senses are inactive, but the never-sleeping Mind,
that pensive, that veiled enchantress, in her mysterious retirement,
looks over the ambrotypes she has collected--ambrotypes, for they are
truly unfading impressions--and, combining them together, as they chance