in relation to the Physical Constitution of the Sun. 67 



2. It became obviously, then, of primary importance — 



i. To study the hydrogen spectrum very carefully under varying 

 conditions, with the view of detecting whether or not there existed a 

 line in the orange, and 



ii. To determine the cause to which the thickening of the F line 

 is due. 



We have altogether failed to detect any line in the hydrogen 

 spectrum in the place indicated, i. e. near the line D ; but we 

 have not yet completed all the experiments we had proposed to 

 ourselves. 



With regard to the thickening of the F line, we may remark that, 

 in the paper by MM. Pliicker and Hittorf, to which reference was 

 made in the communication before alluded to, the phenomena of the 

 expansion of the spectral lines of hydrogen are fully stated, but the 

 cause of the phenomena is left undetermined. 



We have convinced ourselves that this widening out is due to 

 pressure, and not appreciably, if at all, to temperature per se. 



3. Having determined, then, that the phenomena presented by the 

 F line were phenomena depending upon and indicating varying pres- 

 sures, we were in a position to determine the atmospheric pressure 

 operating in a prominence, in which the red and green lines are 

 nearly of equal width, and in the chromosphere, through which the 

 green line gradually expands as the sun is approached*. 



With regard to the higher prominences, we have ample evidence 

 that the gaseous medium of which they are composed exists in a con- 

 dition of excessive tenuity, and that at the lower surface of the chro- 

 mosphere itself the pressure is very far below the pressure of the 

 earth's atmosphere. 



The bulbous appearance of the F line before referred to may be 

 taken to indicate violent convective currents or local generations of 

 heat, the condition of the chromosphere being doubtless one of the 

 most intense action. 



4. We will now return for one moment to the hydrogen spectrum. 

 We have already stated that certain proposed experiments have not 

 been carried out. We have postponed them in consequence of a 

 further consideration of the fact that the bright line near D has ap- 

 parently no representative among the Fraunhofer lines. This fact 

 implies that, assuming the line to be a hydrogen line, the selective 

 absorption of the chromosphere is insufficient to reverse the spec- 

 trum. 



It is to be remembered that the stratum of incandescent gas which 

 is pierced by the line of sight along the sun's limb, the radiation from 

 which stratum gives us the spectrum of the chromosphere, is very 

 great compared with the radial thickness of the chromosphere itself; 

 it would amount to something under 200,000 miles close to the 

 limb. 



Although there is another possible explanation of the non-reversal 

 of the D line, we reserve our remarks on the subject (with which the 

 visibility of the prominences on the sun's disk is connected) until 

 further experiments and observations have been made. 



* Will not this enable us ultimately to determine the temperature ? 



F2 



