366 PROCEEDINGS OF THE AMERICAN ACADEMY. 



and blue of the spectrum formed by the glass prism. The results ob- 

 tained have been applied to the formula, and most excellent agreement 

 found. 



The anomalous dispersion of the vapor was first observed by Kundt, 

 and has since been studied by Becquerel, Julius, Ebert, and others. These 

 investigators have for the most part limited their observations to the dis- 

 persion produced by flames of prismatic form containing the vapor of 

 sodium. Under these conditions the anomalous dispersion is only appar- 

 ent in the immediate vicinity of the D lines, extending to a distance up 

 and down the spectrum not much greater than the distance which sepa- 

 rates the lines. Some four years ago it was shown by the writer that 

 by employing the vapor of the metal in glass tubes, it was possible to 

 obtain far greater deviations, and to extend the observations on the dis- 

 persion from the extreme red to the violet. These results were made 

 possible by the very remarkable physical properties of the vapor, which 

 will be discussed more in detail presently. 



The prisms formed in heated glass tubes are in reality non-homogene- 

 ous cylinders of the vapor, the density being greatest along the heated 

 fioor of the tube, and least along the top. The definition given by non- 

 homogeneous masses of vapor is surprisingly good, and relative values of 

 the refractive indices can be determined without difficulty ; but as no 

 means could be found of determining the angle of the equivalent prism, 

 it was impossible to do more than guess at the absolute values. An at- 

 tempt made several years ago to obtain data by interference methods 

 gave no results, owing to the fact that the fringes were immediately oblit- 

 erated by the convection currents of heated hydrogen. By adopting the 

 expedient of heating the metal in a high vacuum it has been found possi- 

 ble to obtain deviations as high as 1500 fringe-widths with the Dg light 

 of helium, without in any way altering the distinctness of the fringes. 

 By combining the data obtained with the interferometer with the relative 

 values obtained by prismatic deviation, it has been possible to obtain ab- 

 solute values for a very wide range of wave-lengths. We will begin 

 with a brief discussion of the properties of the medium. 



Physical and Optical Proi'krtihs of the Vator of 

 Metallic Sodium. 



One of the chief difficulties which has stood in the way of measuring 

 the optical constants of sodium vapor is the impossibility of confining the 

 vapor in vessels bounded by flat surfaces of glass. As soon as the tem- 

 perature is raised to such a point that vapor of any considerable density 



