256 REPORTS ON INVESTIGATIONS AND PROJECTS. 



The bright lines of the fluorescent spectrum coincide with dark 

 lines of the absorption spectrum, but only a small percentage of the 

 absorption lines are represented. The same has been found to hold 

 true for the bright-line spectrum obtained when plane-polarized light 

 is passed through the .vapor in a magnetic field. (See paper on Mag- 

 neto-Optics of Sodium Vapor.) 



An emission of light of the same wave-length as the exciting light 

 has been observed when the vapor was illuminated with sodium light. 

 This phenomenon may be called resonance radiation, and very pos- 

 sibly differs from fluorescence. It has been repeatedly looked for, 

 but never before observed. It is of considerable importance in con- 

 nection with the theory of the mechanics of absorption. 



The 77iagneto- optics of sodiu?n vapor and the rotatory dispersion 

 fornuda. — The results of the inquiry into this subject were published 

 in the Philosophical Magazine for October. 



The formula given by Drude in his ' ' Optics ' ' for the magnetic 

 rotatory dispersion has been proven by measuring the rotation of the 

 vapor of metallic sodium, formed in exhausted tubes in a powerful 

 magnetic field, and the fact has been established that the numerous 

 absorption lines of the vapor in the red and green-blue region exer- 

 cised powerful rotatory effects. 



In this preliminary work glass tubes were used, which were 

 exhausted and sealed off from the pump. It was subsequently found 

 that the hydrogen liberated from the sodium interfered greatly with 

 the rotatory effects, and in the subsequent work the tubes were kept 

 in connection with the pump. The phenomena exhibited by the 

 vapor are extremely beautiful and very easily shown. 



The results obtained from measurements made with the micrometer 

 are shown in the form of a curve, observed values being represented 

 by circles. Values of the constants A and B in the formula were 

 calculated from two observed values of 5, and the values of ^ for 

 various wave-lengths calculated. These calculated values are rep- 

 resented by crosses on the plate, and will be found to fall almost 

 exactly on the experimental curve. The value of the constant B, 

 which is associated with the absorption line D^, was about double 

 that of A, which belongs to D,. Tables of rotations for various vapor 

 densities were made, and the formula tested under various condi- 

 tions. With various dense vapors the observed value of '5 midway 

 between Dj and D.^ was usually larger than the calculated. Rota- 

 tions as great as 1500° were observed. 



A series of photographic records, taken collectively, exhibit the 

 general form of the rotatory dispersion curve. 



