﻿502 Prof. E. W. Wood on Fluorescence and 



the magnet turned on, the air-pump being worked occasionally 

 to remove the hydrogen which is given off from the sodium. 

 A bright yellow spot will appear on the slit of -the spectroscope, 

 which is seen to be made up of radiations chiefly in the 

 immediate vicinity of the D lines. The phenomena at the 

 D lines have been fully described in the paper already alluded 

 to (Magneto-Optics). 



When the vapour acquires a considerable density, a most 

 magnificent bright-line spectrum appears in the red and 

 green-blue region. Each bright line corresponds to a dark 

 line in the absorption spectrum, but only a small percentage 

 of the dark lines appear to exercise a rotatory power. Some 

 of the strongest absorption-lines are absolutely unrepresented 

 in the magnetic- rotation spectrum, which indicates that there 

 is some radical difference in the absorbing mechanism. 



It is with the bright-line spectrum in the green- blue region 

 that we are now concerned. This spectrum has been photo- 

 graphed with the large, three-prism long-focus spectrograph, 

 and also with the twelve-foot concave grating. Reproductions 

 of the prism spectrograms are given on PI. XIL,/ and m. The 

 magnetic spectrum made with the large grating and the 

 absorption spectrum recorded on the same plate are reproduced 

 on PI. XL, c and d. 



Only about sixty lines appear in this spectrum, in contrast 

 to the 1500 in the absorption spectrum. The intensities are 

 very variable and apparently bear no relation to the intensities 

 of the corresponding absorption-lines. The rotatory lines in 

 many cases coincide with the heads of the groups of absorp- 

 tion-lines, though the centre of the line appears to be slightly 

 displaced beyond the head of the group of absorption-lines. 

 The displacement is, however, very slight, not more than 

 half the width of the line. A list of the wave-lengths of all 

 the lines visible on the negative follows (p. 503). The 

 approximate intensities are represented by numerals, 10 

 indicating the maximum intensity and 1 the minimum. 



At first sight there appears to be no regularity whatever 

 in the distribution of the lines, except perhaps above wave- 

 length 502, where they appear to be about equally spaced in 

 small groups of three or four lines each. Without the aid 

 of the fluorescence spectra of the vapour excited by mono- 

 chromatic light, it is doubtful whether any regular series of 

 lines could be found in the magnetic spectrum, for, as has 

 been subsequently found, more than half of the lines in the 

 series are absent, and there are six or more series present. 

 The fluorescence spectrum with white-light excitation is 

 shown on PI. XI., e, which is from a negative made with the 



