WOOD. — ANOMALOUS DISPERSION OF SODIUM VAPOR. 389 



The observations extended over several weeks, many tubes both of steel, 

 porcelain, and glass being used. The length heated was varied and the 

 different series of observations compared. Most of this work was only use- 

 ful in determining and eliminating the sources of error. The final series, 

 which was considered the best, was made with a tube of Jena glass, the 

 length of the heated portion being 8 cms. The retardation, or rather the 

 acceleration, since we are working with light on the blue side of the 

 absorption band, is due to a column 16 cms. long, since it is twice tra- 

 versed by the interfering beam. From the data given in this table 

 the refractive index of the vajior for either helium light or the green 

 mercury radiation can be calculated for any temperature by means of the 



formula n = \ ± (the -f- or — sign being used according to the direc- 

 tion of the shift), in which m is the fringe shift and c the length of the 

 column (16 cms.). 



Curves are given for these optically determined densities in Plate 2, 



The Refraction and Dispersion of Sodium Vapor of 

 Great Density. 



A knowledge of the absolute value of the refractive index of the 

 vapor, and its dispersion, enables us to compile a table of the refractive- 

 indices for all wave-lengths, for vapors of varying density. This has been 

 done for the very dense vapor obtained by heating a vacuum tube contain- 

 ing the metal to the temperature of 644° C. A column of the vapor 

 at this temperature 8 cms. in length examined by transmitted light has 

 a distinct blue color, as a result of the channelled absorption spectrum. 

 The values are given in the following table, and will be spoken of in 

 future as " observed values " to distinguish them from values calculated 

 from the dispersion formula. It must be remembered that sodium vapor 

 as dense as that with which we are dealing in the present case has an 

 absorption band at the D lines broad enough to completely cut out 

 everything down to and even below the helium line, at least for all 

 thicknesses with which it is possible to work. On this account we are 

 obliged to calculate the refractive indices within this region from obser- 

 vations made with a less dense vapor, a method which in the present case 

 is probably allowable within certain limits. A thin enough sheet of the 

 vapor would probably transmit light within this region with a velocity 

 indicated by the calculated indices. The question of selective reflection 

 at the surface and refusal to transmit the radiation will be considered 

 presently. 



