THE RECENT PROGRESS OF SOLAR PHYSICS. 5 



physical constitution, and perhaps of its past history with that of other 

 suns, and even assumed to give us information whence we might infer 

 something as to their mass, as well as physical constitution, while it 

 has immensely increased the number of lines mapped twenty years 

 since in the spectrum, and modified the ideas we then entertained as 

 to the interpretation of these lines themselves. 



The important question of the amount of heat received from the 

 sun has been the subject of almost uninterrupted experiment and 

 study during the period under review, but without essentially altering 

 the data of Herschel and Pouillet which we already jiossessed. In 

 this field the French physicists and our countryman, Mr. Ericsson, 

 have been prominent workers, and we have attained results possessing 

 all desirable certitude relatively to our knowledge in other branches. 



Investigations on the solar temperature have been carried on by 

 many observers, but with results which ai'e thus far less satisfactory. 



I am painfully sensible of the inadequacy of this review of the 

 history of solar physics, but the brief time before me warns me to 

 come from its past to its present. Within the last two years the difii- 

 culties I have alluded to, as so great in eye-studies of the solar sur- 

 face, have been singularly modified by the remarkable advance of solar 

 photography at the hands of M. Janssen. When I recently visited 

 his observatory at Meudon, I found him producing original negatives 

 on a scale of nearly thirty English inches to the solar diameter, and 

 which bear enlargement to nearly ten feet with remarkable precision ; 

 and one of these negatives, which presents over a million discrete 

 cloud-forms, can be taken in ^-^ of a second. In another branch of 

 photography, that of the reproduction of spectral lines, for which so 

 much is due to Rutherfurd and Draper, I know nothing more surpris- 

 ing than the recent success of Captain Abney (of the Royal Engi- 

 neers) at South Kensington, who has photographed the red end of the 

 spectrum, and far beyond the red end, to a wave-length of about 12,- 

 000. As this statement may of itself convey no clear idea to some of 

 my audience, let me explain in less technical language that it means 

 we can now photograph objects in absolute darkness — objects which 

 are not luminous — simply by the heat they give out. This is a dis- 

 covery which obviously lends itself to important practical applications, 

 while it is of further interest as bringing another proof of that identity 

 of heat and light, with radiations differing only in wave-length, long 

 since surmised by physicists, and asserted prominently by Dr. John 

 Draper, wliose photographs are also the earliest in the path which 

 Captain Abney has carried on by indei^endent methods. Theoretically, 

 there Avould seem to be no limit to this power of photography so long 

 as objects radiate any heat whatever. 



Of recent coronal studies, I have only to speak of the opportunity 

 afforded by the eclipse of last year in our own Western territory. 

 Observed as it was in the pure air of the Rocky Mountains, we found 



