iment was 'that the arc is a medium which emits
the rays D on its own account, and at the same time absorbs them when
they come from another quarter.' Here he stopped. He did not extend
his observations beyond the voltaic arc; he did not offer any
explanation of the lines of Fraunhofer; he did not arrive at any
conception of solar chemistry, or of the constitution of the sun. His
beautiful experiment remained a germ without fruit, until the
discernment, ten years subsequently, of the whole class of phenomena
to which it belongs, enabled Kirchhoff to solve these great problems.

Soon after the publication of Kirchhoff's discovery, Professor Stokes,
who also, ten years prior to the discovery, had nearly anticipated it,
borrowed an illustration from sound, to explain the reciprocity of
radiation and absorption. A stretched string responds to aerial
vibrations which synchronize with its own. A great number of such
strings stretched in space would roughly represent a medium; and if
the note common to them all were sounded at a distance they would take
up or absorb its vibrations.

When a violin-bow is drawn across this tuning-fork, the room is
immediately filled with a musical sound, which may be regarded as the
_radiation_ or _emission_ of sound from the fork. A few days ago, on
sounding this fork, I noticed that when its vibrations were quenched,
the sound seemed to be continued, though more feebly. It appeared,
moreover, to come from under a distant table, where stood a number of
tuning-forks of different sizes and rates of vibration. One of these,
and one only, had been started by the sounding fork, and it was the
one whose rate of vibration was the same as that of the fork which
started it. This is an instance of the _absorption_ of the sound of
one fork by another. Placing two unisonant forks near each other,
sweeping the bow over one of them, and then quenching the agitated
fork, the other continues to sound; this other can re-excite the
former, and several transfers of sound between the two forks can be
thus effected. Placing a cent-piece on each prong of one of the forks,
we destroy its perfect synchronism with the other, and no such
communication of sound from the one to the other is then possible.

I have now to bring before you, on a suitable scale, the demonstration
that we can do with _light_ what has been here done with sound. For
several days in 1861 I endeavoured to accomplish this, with only
partial succ