1869.] in relation to the Physical Constitution of the Sun. 289 



spectrum of this envelope (which has heen named for accuracy of descrip- 

 tion the "chromosphere") the hydrogen line in the green corresponding 

 with Fraunhofer' s line F takes the form of an arrowhead, and widens from 

 the upper to the lower surface of the chromosphere. 



ii. That ordinarily in a prominence the F line is nearly of the same 

 thickness as the C line. 



hi. That sometimes in a prominence the F line is exceedingly brilliant, 

 and widens out so as to present a bulbous appearance above the chromo- 

 sphere. 



iv. That the F line in the chromosphere, and also the C line, extend 

 on to the spectrum of the subjacent regions and re-reverse the Fraunhofer 

 lines. 



v. That there is a line near D visible in the spectrum of the chromo- 

 sphere to which there is no corresponding Fraunhofer line. 



vi. That there are many bright lines visible in the ordinary solar spec- 

 trum near the sun's edge. 



vii. That a new line sometimes makes its appearance in the chromo- 

 sphere. 



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



i. To study the hydrogen spectrum very carefully under varying con- 

 ditions, 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 pressures, we were 

 in a position to determine the atmospheric pressure operating in a promi- 

 nence, 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 condition of ex- 

 cessive tenuity, and that at the lower surface of the chromosphere itself the 

 pressure is very far below the pressure of the earth's atmosphere. 



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



