120 ABSORPTION OF LIGHT. 



been absorbed in their passage from the sun to the earth : 

 although some of them have no doubt undergone ab- 

 sorption within the limits of the earth's atmosphere, we 

 have every reason to believe, with Sir John Herschel, 

 that the principal absorption takes place in the atmos- 

 phere of the sun.* 



It has been proved by Dr. Miller, that the number of 

 those dark lines is continually varying with the altera- 

 tion of atmospheric conditions ;f an d the evidences 

 which have been afforded, of peculiar states of absorp- 

 tion by the gaseous envelope of the earth, during the 

 prosecution of investigations on the chemical agencies 

 of the sun's rays. are of a sufficiently convincing cha- 

 racter. 



It has been calculated by Bouguer, that if our atmos- 

 phere, in its purest state, could be extended rather more 

 than 700 miles from the earth's surface instead of nearly 

 40, as it is at present, the sun's rays could riot penetrate 



* Sir John Herschel, in bis paper On the Chemical Action of the 

 Rays of the Solar Spectrum on Preparations of Silver, remarks that, 

 " it may seem too hazardous to look for the cause of this very sin- 

 gular phenomenon in a real difference between the chemical agen- 

 cies of those rays which issue from the central portion of the sun's 

 disc, and those which, emanating from its borders, have undergone 

 the absorptive action of a much greater depth of its atmosphere; 

 and yet [ confess myself somewhat at a loss what other cause to 

 assign for it. It must suffice, however, to have thrown out the 

 hint; remarking only, that I have other, and, I am disposed to 

 think, decisive evidence (which will find its place elsewhere) of the 

 existence of an a.bsorptive solar atmosphere, extending beyond the 

 luminous one. The breadth of the border, I should observe, is 

 small, not exceeding - 5 or i part of the sun's radius, and this, 

 from the circumstances of the experiment, must necessarily err in 

 excess." Philosophical Transactions, 1840. 



f Experiments and Observations on some Cases of Lines in the 

 Prismatic Spectrum, produced bi/ the passage of Light through 

 Coloured Vapours and Gases and from certuin Coloured Flames. 

 By W. A. Miller, M.D., F.E.S., Professor of Chemistry in King's 

 College, London. Philosophical Magazine, vol, xxvii. 



