Production of a Spectrum by Anomalous Dispersion. 551 



Hence the sufficiency of the bare assumption of hetero- 

 geneous conductivity to explain the double refraction o£ electric 

 waves by a medium like wood can be tested by comparing 

 the value of /.i, obtained by calculation from equation (8), 

 with the dielectric constant of the medium measured by some 

 static method. The latter data I have not at hand. 



However, without such a comparison, it is still seen that 

 heterogeneous conductivity plays an important part in the 

 phenomenon of double refraction of electric waves. 



Universitat Leipzig-. 

 January, 19.01. 



LII. On the Production of a Bright-Line Spectrum by Ano- 

 malous Dispersion and its Application tlie "Flash-Spectrum." 

 By R. W. Wood * 



IN a communication published in the Proceedings of the 

 Royal Academy of Sciences, Amsterdam, W. H. Julius 

 makes the very brilliant suggestion that the " flash-spectrum," 

 seen immediately at totality, may be due to photosphere-light 

 abnormally refracted in the atmosphere of metallic vapours 

 surrounding the sun : in other words, the light of the flash- 

 spectrum does not come from the reversing layer at all, but 

 from the photosphere. The author shows that the light which 

 will be thus abnormally refracted w^ill be of wave-lengths 

 almost identical with the wave-lengths which the metallic 

 vapours are themselves capable of radiating. This beautiful 

 theory not only explains the apparent shallowness of the 

 reversing layer, a thing that has always puzzled astro- 

 physicists, but it accounts for the extraordinary brilliancy of 

 the lines. 



I have succeeded in producing such a flash-spectrum by an 

 arrangement in which I have endeavoured to imitate as 

 closely as possible the conditions supposed to exist at the 

 surface of the sun : in brief, I have obtained a spectrum of 

 bright lines, with light from a source showing a continuous 

 spectrum, by means of anomalous dispersion in an incan- 

 descent metallic vapour. 



The theory of Julius supposes the sun to be surrounded by 

 an atmosphere of metallic vapours, the refractive index of 

 which decreases with increasing distance from the surface. 

 In this atmosphere the rays of light coming from the photo- 

 sphere will move in curved paths similar to rays in our own 

 atmosphere. 



The refractive index is, however, very small except for 

 * Communieated by the Physical Society : read April 26, 1901. 



