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 success.  In iron dishes a mixture of dilute alcohol and salt was placed, and warmed so as to promote vaporization.  The vapour was ignited, and through the yellow flame thus produced the beam from the electric lamp was sent; but a faint darkening only of the yellow band of a projected spectrum could be obtained.  A trough was then made which, when fed with the salt and alcohol, yielded a flame ten feet thick; but the result of sending the light through this depth of flame was still unsatisfactory.  Remembering that the direct combustion of sodium in a Bunsen’s flame produces a yellow far more intense than that of the salt flame, and inferring that the intensity of the colour indicated the copiousness of the incandescent vapour, I sent through the flame from metallic sodium the beam of the electric lamp.  The success was complete; and this experiment I wish now to repeat in your presence.[25]