The Anomalous Dispersion of Sodium Vapour. 161 



region in the blue almost entirely. Meanwhile the curvature of the 

 spectrum increases in a most remarkable manner, and the entire red end 

 is lifted high above the green-blue end. As the density of the vapour 

 increases the red gradually fades away, leaving only the yellow and 

 green and the remote blue and violet, the curvature increasing all the 

 while. The fluted or channelled spectrum was described by Roscoe 

 and Schuster about twenty-five years ago, but so far as I know no 

 work has been done on it since. I have recently secured excellent 

 photographs of it with a Eowland concave grating, from the extreme 

 red to the violet, and find that it is much more extensive than has been 

 supposed, for the flutings run right up to the absorption band at the 

 D lines on both sides, though they are very faint on the side of 

 shorter wave-lengths. This spectrum will be described in a subsequent 

 paper. 



Very satisfactory photographs of the dispersed grating spectrum 

 were secured, some of which are reproduced. It was found impossible 

 to maintain a sufficiently uniform density, at the low temperature, for 

 a sufficient length of time to enable a negative to be secured showing 

 the appearance before the light between the D lines vanished. I 

 therefore went back to the old plan of using a prismatic flame. After 

 some experimenting it was found that the most satisfactory flame was 

 secured by passing hydrogen through a tube containing metallic 

 sodium, strongly heated, and burning the gas at a flat jet piece 

 made of platinum foil. An exceedingly dense and very uniform 

 sodium flame is obtained in this way, which can be maintained almost 

 indefinitely. The arrangement of the lamp is shown in fig. 3, the 

 diagram requiring no description. 



