a Bright-Line Spectrum by Anomalous Dispersion. 553 



Cutting off the sunlight with a screen caused the instant 

 disappearance of the bright lines. 



F«r. 1. 



I fiAsrf/? Plate I 



'« CAPSULE 



s «s? 



/D,~S8952 \ 

 VA> = S890-2 / 



Fig. 2. — Flash Spectrum of Sodium produced by Anomalous 

 Dispersion. 



Repeating the experiment, I found that the bright lines 

 came into view on the sides of the sodium lines towards the 

 blue ; that is to say, it is light for which the medium has an 

 abnormally low refractive index that is bent around the edge 

 of the plate and enters the instrument, This is precisely 

 what we should expect, for sodium vapour has a refractive 

 index of less than 1 for waves slightly shorter than Dj and 

 D 2 , as was shown by Julius in his paper * The rays then will 

 be concave upward in a medium in which the refractive index 

 varies, as I have supposed it to vary in the present case. If 



* I have since found that the refractive index is less than If or the entire 

 vellow, green, and blue portion of the spectrum, and greater than 1 for 

 the entire red, orange, yellow end. In other words, I have obtained a 

 complete anomalous spectrum with sodium vapour. 



Phil. Mag. S. G. Vol. 1. No. 5. May 1901. 2 



