410 



Dr. W. Huggins. On a Method of [Dec. 21. 



The spectroscopic method of viewing the solar prominences fails, 

 because a large part of the coronal light gives a continuous spectrum. 

 The successful photograph of the spectrum of the corona taken in 

 Egypt, with an instrument provided with a slit, under the super- 

 intendence of Professor Schuster during the solar eclipse of May 17, 

 1882, shows that the coronal light as a whole, that is the part which 

 gives a continuous spectrum, as well as the other part of the light 

 which may be resolved into bright lines, is very strong in the region 

 of the spectrum extending from about Gr to H. It appeared to me, 

 therefore, very probable that by making exclusive use of this portion 

 of the spectrum it might be possible under certain conditions, about to 

 be described, to photograph the corona without an eclipse. 



In the years 1866-68 I tried screens of coloured glasses and other 

 absorptive media, by which I was able to isolate certain portions of 

 the spectrum, with the hope of seeing directly, without the use of the 

 prism, the solar prominences.* I was unsuccessful, for the reason that 

 I was not able by any glasses or other media to isolate so very 

 restricted a portion of the spectrum as is represented by a bright line. 

 This cause of unsuitableness of this method for the prominences, 

 which gi^e bright lines only, recommends it as very promising for the 

 corona. If by screens of coloured glass or other absorptive media the 

 region of the spectrum between Gr and H could be isolated, then 

 the coronal light which is here very strong would have to contend onl} r 

 with a similar range of refrangibility of the light scattered from the 

 terrestrial atmosphere. It appeared to me by no means improbable 

 that under these conditions the corona would be able so far to hold its 

 own against the atmospheric glare, that the parts of the sky imme- 

 diately about the sun where the corona was present would be in 

 a sensible degree brighter than the adjoining parts where the 

 atmospheric light alone was present. It was obvious, however, that 

 in our climate and low down on the earth's surface, even with the aid 

 of suitable screens, the addition of the coronal light behind would be 

 able to increase but in a very small degree the illumination of the 

 sky at those places where it was present. There was also a serious 

 drawback from the circumstance that although this region of the 

 spectrum falls just within the range of vision, the sensitiveness of the 

 eye for very small differences of illumination in this region near its limit 

 of power is much less than in more favourable parts of the spectrum ; 

 at least such is the case with my own eyes. There was also another 

 consideration of importance ; the corona is an object of very complex 

 form, and full of details depending on small differences of illumina- 

 tion, so that even if it could be glimpsed by the eye, it could scarcely 

 be expected that observations of a sufficiently precise character could 



* " Monthly Notices," vol. xxviii, p. 88, and vol. xxix, p. 4. 



