Proceedings. 39 



added to the four luminous spectra obtained by various 

 spectroscopists at different temperatures and pressures, 

 apparently made a total of six spectra of this one gas. 

 Janssen states that the two absorption spectra are pro- 

 ducible separately and independently, one being the line 

 spectrum in the A, B, and a region, that is, in the red and 

 orange-red, and the other a spectrum of bands in the 

 red, orange-green, and blue. The intensity of the former 

 spectrum varies simply with the product of the thickness 

 of gas traversed by the light, and the density ; whereas 

 the intensity of the band spectrum varies according to 

 the thickness and the square of the density. From the 

 fact that the assumed corresponding dark lines and bands 

 observed in the solar spectrum seemed to obey these 

 laws, when examined from the Grands Mulcts station on 

 Mont Blanc, at an altitude of 10,000 feet, the bands 

 being absent and the lines weakened proportionately, 

 Janssen infers that their presence and relatively greater 

 intensity in the solar spectrum when observed at lower 

 levels are undoubtedly due to the greater thickness and 

 density of the atmospheric oxygen traversed, and hence 

 that they are telluric lines and bands and in no way 

 indicative of the existence of solar oxygen. Referring to 

 the statement that Janssen's absorption bands occur in the 

 red, orange-green, and blue, Mr. Faraday pointed out that 

 Plucker's bright oxygen spectrum, which has been called the 

 " compound line " spectrum, of which a corresponding 

 reversal spectrum has been, it is believed, identified in the 

 solar spectrum, occurs in the red, green, and blue. Professor 

 Henry Draper's supposed bright band solar oxygen spectrum 

 was photographed in the blue, and there also are the dark 

 absorption lines by which these bright bands were subse- 

 quently found to be traversed, and which Professor J. C. 

 Draper suggested might be the reversal lines of oxygen. 

 Finally in the red and orange-green the absorption lines 



