Scientific Lectures. 165 



gives US light of tlie sodium tint. Litliium did not give ua light of 

 that tint. The sodium light of the white flame will be wholly cut 

 off by the sodium flame ; for the yellow light you see is' simply the 

 yellow light of the sodium flame ; and all the other colors will pass 

 through the flame. Suppose, now, we direct a spectroscope like this, 

 in such a way that we look through the sodium flame toward the 

 white light, what shall we see? "We shall see, in the first place, a con- 

 tinuous spectrum of blending colors. In the next place we shall see 

 the sodium band projected, as we term it, upon the spectrum. Let 

 us now alter the conditions slightly, and instead of this small gas 

 burner use the brilliant electric light, as I shall presently show you, 

 and then direct our spectroscope through the sodium flame toward 

 the electric light, we shall then have the same result as before ; that 

 is, we shall see the continuous spectrum, and shall also see the sodium 

 band ; but now the sodium band will be so much darker than the 

 brilliant spectrum produced by this exceedingly powerful source of 

 light that it will appear black ; and the result will be a black band 

 crossing the spectrum in the position of the bright sodium bands. 

 [Applause.] You see, therefore, that the black band and the bright 

 band were precisely the same thing ; that they mean simply a sodium 

 light ; the band appearing light or dark by contrast with the rest of 

 the spectrum. The sodium band is illuminated only by the sodium 

 flame. The rest of the flame is illuminated by the electric light ; 

 and the electric light being far more brilliant, of course the less 

 brilliant sodium band appears dark by contrast. If, then, we see 

 from any source of light a continuous spectrum crossed by a dark 

 line in the position of the sodium band, we know that we are look- 

 ing toward a solid source of light, but that that light from an incan- 

 descent solid, before it reaches our instrument has passed through 

 sodium vapor. I have spoken thus far of the sodium band as 

 a single one, and with an instrument of this size it appears 

 as a single one ; but if we use a powerful instrument that sodium 

 band is found to be double. It consists of two bands, only those two 

 bands are very close together. If we turn this instrument toward tho 

 sun, we see a continuous spectrum, and that continuous spectrum 

 is crossed by a dark line in precisely the position of the sodium 

 band. Moreover, if an instrument has suflicient power, like 

 that we have in Cambridge, that band appears as a double line, and 

 each of its component parts coincides exactly with the position of the 

 double sodium line. What, then, is the conclusion? That the sun, 



