WOOD. 



ANOMALOUS DISPERSION OF SODIUM VAPOR. 



385 



trograpli. The bottom of the tube having been raised to a full red heat 

 in the combustion furnace, the photographic plate was exposed with the 

 slotted slide in position. The anomalously dispersed spectrum was thus 

 recorded in small sections corresponding to the slits in the slide. The 

 tube was then allowed to cool off, the slide drawn out a distance equal 

 to the width of one of the slits, and a second exposure made which re- 

 corded the position of the uudeviated spectrum. A drawing of the plate 

 holder with its perforated slide, and the deviated and undeviated spectra 

 obtained with it are shown in Fiirures G and 7. 



FiGuiiE 0. 



FioauE 7. 



Absolutk Detkkjiin'atioxs of tiik Rkfuactive Index. 



Tlie deviations of the light rays by the non-homogeneous cylinders 

 of sodium vapor furmed in exhausted tubes give us no measure of the 

 absolute refractive index, for the reason that we have no way of deter- 

 mining the angle of the equivalent prism. Various methods were tried, 

 designed to give to a mass of vapor of uniform density a prismatic form. 

 None of them was very satisfactory, and until the discoveiy was made 



VOL. XL. — 25 



