WOOD. — ANOMALOUS DISPERSION OF SODIUM VAPOR. 377 



might get data for both lines. This experiment would have been tried, 

 were it not for the fact that almost as satisfactory results can be obtained by 

 the method of prismatic deviation, with a much less expenditure of time. 

 As a discussion of the modifications of the appearance of the fringes 

 formed by sodium light, by the introduction of the vapor of sodium into 

 the path of one of the interfering beams of light, has no especial bearing 

 upon the determination of the dispersion of the vapor, it will be postponed 

 for the present. A fuller discussion will be given in a subsequent paper 

 on "The achromatization of fringes formed by approximately homogene- 

 ous light by highly dispersing media." * 



Prismatic Determination of the Dispersion in the Immediate 

 Vicinity of the D Lines. 



As I have already shown, prisms of excellent defining power can 

 be formed by heating metallic sodium in highly exhausted tubes of hard 

 glass. In my earlier experiments, in which the metal was heated in an 

 atmosphere of hydrogen, the definition was not good enough to obtain a 

 smooth curve close to and between the D lines by the method of crossed 

 prisms. The photographs illustrating my first paper, in which the disper- 

 sion between the lines was shown, were taken by employing a prismatic 

 flame of hydrogen burned at a platinum jet, and fed with the vapor of 

 sodium. By using vacuum tubes heated by very small flames far better 

 results can be obtained, and perfectly smooth sharp curves obtained. 

 Two different methods were used in the determination of the deviation 

 produced by the prism for wave-lengths close to the D lines. 



A reticulated mesh was ruled on a small plate of glass with one of the 

 Rowland dividing engines. This plate was mounted in the eye-piece of 

 the spectroscope, on the slit of which the image of an illuminated hori- 

 zontal slit was thrown, after dispersion by the sodium tube. The spec- 

 troscope was provided with a plane grating ruled with twenty thousand 

 lines to the inch, which gave a very brilliant 2d order spectrum. There 

 were four squares of the reticulated mesh between the D lines, and nine 

 squares between Dj and D3. 



As the temperature of the sodium tube was raised the relative deviations 

 in the different squares of the mesh were noted. The light immediately 

 adjacent to the D lines was deviated entirely out of the field of the 

 telescope before any measurable deviation at the helium line occurred. 

 At a temperature at which the spectrum at the helium line was deviated 



* Phil. Mag., September, 1004. 



