238 PROCEEDINGS OF THE AMERICAN ACADEMY. 



slit of a spectroscope. With the steel tube cooled below the point at 

 which sodium vapor forms, the nicols are set for complete extinction, 

 and the field of the spectroscope becomes dark. The tube is now heated 

 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 vapor 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 appears to exercise a rotatory 

 power. Some of the strongest absorption lines are absolutely unrepre- 

 sented 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 photographed with the 

 large, three-prism long focus spectrograph, and also with the twelve-foot 

 concave grating. Reproductions of the prism spectrograms are given 

 on Plate 2, / and m. The magnetic spectrum made with the large 

 grating and the absorption spectrum recorded on the same plate are 

 reproduced on Plate 1, 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 absorption lines, though the centre of the line 

 appears to be slightly displaced beyond the head of the group of ab- 

 sorption 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 is given on page 239. 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 vapor excited by monochromatic 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 



