The. Anomalous Dispersion of Sodium Vapour. 169 



sary before deciding whether the wave-length of the emitted light is 

 greater than that of the exciting vibrations. My impression is that 

 the bright narrow band in the yellow is slightly on the red side of 

 the D lines, but I do not feel absolutely sure of it. It will be 

 interesting to see whether the fluorescence persists for an appreciable 

 time after the light is cut off, which can be easily determined with 

 an especially designed phosphoroscope, a matter which I expect to 

 take up next. 



A quantitative study of the absorption of the vapour is extremely 

 desirable, though the experimental difficulties will be very great. As 

 I have already said, the width of the band at the D lines is often 

 from ten to fifteen times the distance between the lines. Julius has, 

 however, called attention to the fact that we must be on our guard 

 against attributing the absence of light in the spectrum to absorp- 

 tion, when the conditions are such that the rays may have been 

 merely turned to one side by refraction. This he believes, if I read 

 his paper rightly, is the true explanation of the widening of the 

 D lines in the absorption spectrum. To eliminate the possibility of 

 this lateral deviation it is necessary, if we are working with a non- 

 homogeneous medium, to arrange matters so that the rays of light 

 are perpendicular to the surfaces of equal refractive index, instead 

 of parallel, as is the case in the dispersion tubes. One obvious 

 method of getting around the difficulty would be to vaporise the 

 sodium in a vacuum, thus doing away with the variations in the 

 density, but this necessitates contact between the corrosive vapour and 

 the glass. A better plan appeared to be to send the light through 

 the stratified vapour in such a direction that no lateral deviation 

 could result, a matter of some difficulty until the expedient occurred to 

 me of using the surface of the molten metal as a reflector, the rays thus 

 twice traversing the non-homogeneous medium in a direction perpen- 

 dicular to the equi-indicial surfaces. 



A photograph of the absorption spectrum of the vapour in the red 

 and yellow region, obtained with a 10-foot concave grating, is repro- 

 duced on Plate 3, fig. 12. The D lines were photographed just below 

 the spectrum for comparison. The flutings in the red are well shown 

 in this picture, which is but one of a large number that have been 

 taken. Measurements of wave-length have not yet been made, as I 

 feel confident of getting better plates in the near future. The fine 

 dark lines can be found on both sides of the heavy band at the 

 D lines, and I have found that they are present throughout the 

 entire spectrum. An enlargement of a portion of fig. 12 is shown in 

 fig. 13, which gives a better idea of the appearance of the fluted bands. 



On increasing the density of the vapour the red end of the spectrum 

 finally disappears, and by employing a very powerful light in combina- 

 tion with the method above alluded to of reflecting the light from the 



