246 PROCEEDINGS OF THE AMERICAN ACADEMY. 



possible, however, by comparing the spectrum excited by copper with 

 the one excited by zinc 4811, to make a guess as to which lines belong 

 together. 



We will begin by a study of the spectra excited by the cadmium 

 radiations. 



Cadmium Stimulation. 



Photographs of the fluorescent spectrum obtained with the cadmium- 

 arc excitation are shown on Plate 2, h, i, j, k. 



Of these, i and k are excited by all three cadmium lines ; the former 

 taken with a much narrower slit than the latter. Spectrum h was taken 

 when the sodium vapor was excited by the line 480 ; observed visually, 

 it consists of twelve bright lines, in groups of two and four, as shown 

 on the chart immediately above the complex spectrum at the bottom. 

 Midway between the groups very faint lines can be perceived if the eye 

 is carefully rested. The strong lines are arranged thus: two, absent 

 line, four, absent line, two, absent line, four. The absent lines, or, 

 more correctly speaking, the faint lines, evidently belong to the same 

 series, and taken collectively the lines will be found to be very nearly 

 equidistant, measured along a normal spectrum. 



The wave-lengths of the lines in this series, as determined from 

 measurements of the plates obtained with the prism spectrograph, are 

 as follows : 



These wave-lengths I consider to be accurate to within about 2 A. E. 

 or \ of the distance between the D lines. 



It is, of course, of the utmost importance to determine the law which 

 governs the spacing of the lines in the simple spectra. A criterion 

 may perhaps be obtained by referring to the magnetic rotation 

 spectrum of the vapor, the lines of which correspond in general to the 

 lines of the fluorescence spectrum. This spectrum has been photo- 

 graphed with a large concave grating and the wave-lengths determined 

 certainly to within a tenth of an Angstrom unit. The strong lines at 



