WOOD. — ANOMALOUS DISPERSION OF SODIUM VAPOR. 371 



captive by the sodium vapor, condensing witli it as the tube cools.* Wlieu 

 the tube is again heated this occluded hydrogen is evolved with the first 

 traces of sodiuui vapor, and modifies its refractivity. If we are working 

 with light on the same side of the D lines as the helium line, for which 

 the refractive index of the vapor is less than unity, the evolution of this 

 gas will oppose the displacement of the fringes, holding both sets back 

 by practically the same amount. Suppose we are working with a wave- 

 lengtii for which the relative shift with respect to helium light is 1 : 10, 

 If we form a sufficient ([iiuntity of sodium vapor to shift the helium 

 fringes througli 30 fringe-widths, and the other set 3 widths, and an 

 amount of hydrogen suificient to give a shift of one fringe-width has been 

 set free, it is obvious that our observed ratio will be 2 : 29, or in round 

 uumbers, 1 : 15. The errors observed were always at the beginning of 

 the series when the tube was heated, and at the end when it was cooled, 

 and were always of such a nature as to be explained perfectly by the 

 hypothesis which I have advanced. On this account we always dis- 

 regarded the relative values found when the sodium vapor was only 

 present in very small quantities. 



Many weeks of work were necessary before all sources of error were 

 eliminated, and the best methods of working discovered. As a sample 

 of one of the troubles met and overcome, the following will serve. When 

 working with light from the monochromatic illuminator of a wave-length 

 very near that of the D lines the fringes disappeared before shifts of 

 more than one or two fringe-widths were observed. It had been our 

 habit to set the mirrors of the instrument in such positions that the centre 

 of the fringe system was seen in the telescopes. The disappearance was 

 finally. found to be due to the dispersion of the vapor, and by turning the 

 screw of the instrument in such a way as to increase the length of the air 

 path, they appeared again. The most advantageous way to work was 

 therefore to begin the observations with the maximum air path consistent 

 with visibility of the fringes; as the sodium vapor formed they increased 

 in distinctness at first, and then gradually faded away. In this way it 

 was possible to get a shift twice as great as before ; if it was desimble to 

 work with a denser vapor to eliminate the error due to evolved hydro- 

 gen, we have only to increase the length of the air path still further, 

 and work further out in the system. Under these conditions the fringes 



* It is interesting- to compare this with the aberrant solution of iodine vapor in 

 the vapor of CS.,. Wood. Piiil. Mag., 1800 : On the Absorption Spectrum of Solu- 

 tions of Iodine and Bromine above the Critical Temperature. 



