en fully developed, largely
on account of its cost. But those whose memories go back to the
tinder-box period, and recollect the cost of the first lucifer
matches, will not despair of the results of the application of science
and ingenuity to the cheap production of anything for which there is a
large demand.

The influence of the progress of electrical knowledge and invention
upon that of investigation in other fields of science is highly
remarkable. The combination of electrical with mechanical contrivances
has produced instruments by which, not only may extremely small
intervals of time be exactly measured, but the varying rapidity of
movements, which take place in such intervals and appear to the
ordinary sense instantaneous, is recorded. The duration of the winking
of an eye is a proverbial expression for an instantaneous action;
but, by the help of the revolving cylinder and the electrical
marking-apparatus, it is possible to obtain a graphic record of such
an action, in which, if it endures a second, that second shall be
subdivided into a hundred, or a thousand, equal parts, and the state
of the action at each hundredth, or thousandth, of a second exhibited.
In fact, these instruments may be said to be time-microscopes. Such
appliances have not only effected a revolution in physiology, by the
power of analysing the phenomena of muscular and nervous activity
which they have conferred, but they have furnished new methods of
measuring the rate of movement of projectiles to the artillerist.
Again, the microphone, which renders the minutest movements audible,
and which enables a listener to hear the footfall of a fly, has
equipped the sense of hearing with the means of entering almost as
deeply into the penetralia of nature, as does the sense of sight.

[Sidenote: Photography as an instrument of science.]

That light exerts a remarkable influence in bringing about certain
chemical combinations and decompositions was well known fifty years
ago, and various more or less successful attempts to produce permanent
pictures, by the help of that knowledge, had already been made. It was
not till 1839, however, that practical success was obtained; but the
'daguerreotypes' were both cumbrous and costly, and photography would
never have attained its present important development had not the
progress of invention substituted paper and glass for the silvered
plates then in use. It is not my affair to dwell upon the practical
ap