REPORT OF EXECUTIVE COMMITTEE. 1 29 



sodium, which has made possible an experimental verification of the 

 simplest form of the electro-magnetic dispersion formula 



71^ = 1 + 



X' — Xm^ 



This formula has never been tested, for the reason that no data 

 have ever been obtained of the dispersion of a medium in which the 

 velocity of light of different wave-lengths is dependent on the 

 presence of a single absorption band. The dispersion of the vapor 

 was measured b}^ observing the shifts of the interference fringes in 

 a INIichelson interferometer when a given quantity of the vapor w^as 

 introduced into one of the optical paths of the instrument. Usually 

 two sources of monochromatic light were used simultaneously. 

 When working close to the absorption band it was necessary to have 

 lights of very nearh^ the same wave-length, which was accomplished 

 by placing a helium tube in a powerful magnetic field and utilizing 

 the resulting Zeeman double-line for illuminating the interferometer. 

 Absolute determinations were made of the refractive index of the 

 vapor formed in highly exhausted tubes of steel and porcelain at 

 different temperatures, the temperature being determined by means 

 of a thermo-couple of iron and constantin. 



For light of wave-lengths very nearly that of the D lines the re- 

 fractive index of the vapor at a temperature of 650° C. was found to 

 be 1.38 for the wave-length on the red side of the absorption band 

 and 0.62 for light on the blue side. 



Numerical values were obtained for the refractive index from the 

 extreme red to the remote ultra-violet, and the observed values were 

 compared with the values calculated from .the dispersion formula, 

 most excellent agreement being found. 



The vapor was found to have some very remarkable physical prop- 

 erties, which are at the present time under investigation. It appears 

 to have the property of cohesion and perhaps surface tension. A 

 dense mass of it can be formed in the center of a highly exhausted 

 tube, bounded at each end by a vacuum, there being only a very 

 slight amount of diffusion toward the colder parts of the tube. 



The results of the work appear in the Proceedings of the American 

 Academy and the Philosophical Magazine for September, 1904. 



The Fbiorcscetice of Sodium Vapor. — The work which was com- 

 menced in the spring of 1903 on the remarkable fluorescence of so- 

 dium vapor was continued during the following autumn. It was 

 found practicable to photograph the fluorescence spectrum of the 



