( 213 ) 



Physics. — "Anomalous refraction phenomena investigated with the 

 Spectroheliograpk." By Prof. W. H. Julius. 



According to the current interpretation of spectroheliograph results, 

 dark flocculi indicate regions on the sun, where the special gas, a 

 line of which is used, exists in such conditions of density and tem- 

 perature, that it strongly absorbs the light coming from deeper 

 layers, whereas bright flocculi show us regions where, in consequence 

 of higher temperature or chemical or electrical causes, the radiation 

 of the gas exceeds its absorbing effect. 



A few years ago 1 proposed an entirely different explanation of 

 the same phenomena 1 ). An attempt was made to account for the 

 peculiar distribution of the light in photographs, secured with the 

 spectroheliograph, by simply considering the anomalous refraction 

 which waves from the vicinity of the absorption lines must suffer 

 when passing through an absorbing medium, the density of which 

 is not perfectly uniform. 



If it proves possible to explain the observed facts on this basis, 

 we shall be able to dispense with the assumption of any very 

 marked differences as to the absorbing and emitting conditions of a 

 certain gas or vapour in contiguous regions on the sun. Moreover, 

 we then might assume the constituents of the solar atmosphere to be 

 thoroughly mixed, their proportions in the mixture only varying 

 with the distance from the sun's centre. 



That our interpretation does not presuppose the separate existence 

 of cloud-like masses of calcium or iron vapour or of hydrogen, 

 simplifies the conception of the solar body, and therefore looks like 

 an advantage; but even if one were compelled, by other considera- 

 tions, still to believe in the real existence of such separate luminous 

 or dark accumulations of certain substances, it would nevertheless 

 be necessary to consider the effect, which anomalous dispersion of 

 light in those masses must have on the appearances revealed by 

 the spectroheliograph. 



Among the advantages I derived from a visit to the Mount Wilson 

 Solar Observatory, in August 1907, was the opportunity of using 

 the 5 foot spectroheliograph') for some experiments on anomalous 

 refraction. 



It was expected that when light, coming from a source with a 

 continuous spectrum, traverses a space in which sodium vapour is 



1 ) Proc. Roy. Acad. Amsterdam VII, p. 140, (1904;. 



2 ) Hale and Ellerman. "The five-foot spectroheliograph of the Solar Observatory." 

 Contributions from the Solar Observatory Mount Wilson, California, N'. 7. . 



