Firstly then you notice, when a fragment of sodium is placed in a platinum spoon and introduced into a Bunsen’s flame, an intensely yellow light is produced.  It corresponds in refrangibility with the yellow band of the spectrum.  Like our tuning-fork, it emits waves of a special period.  When the white light from the electric lamp is sent through that flame, you will have ocular proof that the yellow flame intercepts the yellow of the spectrum; in other words, that it absorbs waves of the same period as its own, thus producing, to all intents and purposes, a dark Fraunhofer’s band in the place of the yellow.

In front of the slit (at L, fig. 56) through which the beam issues is placed a Bunsen’s burner (b) protected by a chimney (C).  This beam, after passing through a lens, traverses the prism (P) (in the real experiment there was a pair of prisms), is there decomposed, and forms a vivid continuous spectrum (S S) upon the screen.  Introducing a platinum spoon with its pellet of sodium into the Bunsen’s flame, the pellet first fuses, colours the flame intensely yellow, and at length bursts into violent combustion.  At the same moment the spectrum is furrowed by an intensely dark band (D), two inches wide and two feet long.  Introducing and withdrawing the sodium flame in rapid succession, the sudden appearance and disappearance of the band of darkness is shown in a most striking manner.  In contrast with the adjacent brightness this band appears absolutely black, so vigorous is the absorption.  The blackness, however, is but relative, for upon the dark space falls a portion of the light of the sodium flame.

[Illustration:  Fig. 56.]

I have already referred to the experiment of Foucault; but other workers also had been engaged on the borders of this subject before it was taken up by Bunsen and Kirchhoff.  With some modification I have on a former occasion used the following words regarding the precursors of the discovery of spectrum analysis, and solar chemistry:—­’Mr. Talbot had observed the bright lines in the spectra of coloured flames, and both he and Sir John Herschel pointed out the possibility of making prismatic analysis a chemical test of exceeding delicacy, though not of entire certainty.  More than a quarter of a century ago Dr. Miller gave drawings and descriptions of the spectra of various coloured flames.  Wheatstone, with his accustomed acuteness, analyzed the light of the electric spark, and proved that the metals between which the spark passed determined the bright bands in its spectrum.  In an investigation described by Kirchhoff as “classical,” Swan had shown that 1/2,500,000 of a grain of sodium in a Bunsen’s flame could be detected by its spectrum.  He also proved the constancy of the bright lines in the spectra of hydrocarbon flames.  Masson published a prize essay on the bands of the induction spark; while Van der Willigen, and more recently Pluecker, have also given us beautiful drawings of spectra obtained from the same source.