When once the availability of one great primitive agent is worked out,
it is easy to foresee how extensively it will assist in unravelling
other secrets in natural science. The simple principle of the
Stereoscope, for instance, might have been discovered a century ago,
for the reasoning which led to it was independent of all the properties
of light; but it could never have been illustrated, far less multiplied
as it now is, without Photography. A few diagrams, of sufficient
identity and difference to prove the truth of the principle, might
have been constructed by hand, for the gratification of a few sages;
but no artist, it is to be hoped, could have been found possessing
the requisite ability and stupidity to execute the two portraits, or
two groups, or two interiors, or two landscapes, identical in every
minutia of the most elaborate detail, and yet differing in point of
view by the inch between the two human eyes, by which the principle is
brought to the level of any capacity. Here, therefore, the accuracy and
insensibility of a machine could alone avail; and if in the order of
things the cheap popular toy which the stereoscope now represents was
necessary for the use of man, the photograph was first necessary for
the service of the stereoscope.--_Quarterly Review_, No. 202.