

Mr. G. J. Stoney on the Solar Eclipse, August 1868. 503 



August 1868, and for which there will not occur so good an op- 

 portunity for several years afterwards. An abstract of this com- 

 munication has been published in the Proceedings of the Royal 

 Society, and reprinted in the Philosophical Magazine ; but the 

 memoir and its appendix are not yet in type. As so great a 

 delay has occurred, I fear lest the suggestions to which I have 

 referred may not appear in time to be of use, and I therefore 

 venture to submit them to the Royal Astronomical Society. 



In the memoir it is shown that the gases which constitute the 

 solar atmosphere range to unequal heights, and that the order in 

 which the outer boundaries of those which are known succeed 

 one another is probably the following : — hydrogen extending 

 furthest ; then, in order after it, sodium and magnesium ; cal- 

 cium; chromium, manganese, iron, cobalt, and nickel; and 

 lastly, copper, zinc, and barium. These, and whatever other 

 gases exist in the sun's atmosphere, intercept all light coming 

 from beyond them of the refrangibilities which correspond to 

 their spectral rays, and substitute for it the feebler light which 

 emanates from their own upper strata. In this way they pro- 

 duce those dark lines in the spectrum which are so familiar, and 

 the various intensities of which depend in great measure upon 

 the different temperatures of the upper layers of the gases in 

 which they have their origin. 



Now, if the corona which is seen during a total eclipse is 

 caused, as we must presume, by the sun's enormous outer atmo- 

 sphere projecting beyond the disk of the moon, the light which 

 reaches us from it is probably in part borrowed light, and in part 

 due to the atmosphere being itself self-luminous. Borrowed 

 light, however it may originate*, will give a spectrum resembling 

 the sun's, whereas any light emitted by the solar atmosphere in 

 virtue of its being incandescent will consist of bright rays in the 

 same positions as some of Fraunhofer's lines. Accordingly, when 

 examined through a spectroscope adapted to an equatorial tele- 

 scope f, much of the light of the corona may be found to resolve 



* Borrowed light may come either from a mist of solid or liquid par- 

 ticles in the sun's atmosphere, or because some of the gases are sensibly- 

 coloured, or from irregular refractions occasioned by the troubled condition 

 of the sun's atmosphere, but will not in any considerable degree arise from 

 the illumination of the earth's atmosphere. It is from the absence of this 

 last and chief source of borrowed light that the corona comes into view, 

 and observations during an eclipse are so precious. 



t The equatorial stand is not essential, as it would be enough to pro- 

 vide a side reflector with a slit in it, through which the rays to be observed 

 in the spectroscope shall pass ; and the rest of the image being viewed in 

 the reflector by an assistant, he could by hand movements sufficiently direct 

 the telescope. A telescope of considerable aperture and focal length would 

 be best. 



