SECT, xxiv.] DAGUERREOTYPE. 235 



striking contrast of colour with the interior spectrum, the 

 former being lead gray, the latter an extremely rich deep 

 velvety brown. The less refrangible end of this interior brown 

 spectrum presented a sharply terminated and regularly ellip- 

 tical contour, the more refrangible a less decided one. " It 

 may seem too hazardous," Sir John continues, "to look for the 

 cause of this very singular phenomenon in a real difference 

 between the chemical agencies of those rays which issue from 

 the central portion of the sun's disc, and those which, ema- 

 nating from its borders, have undergone the absorptive action 

 of a much greater depth of its atmosphere ; and yet I 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 of the existence of an absorptive solar atmosphere 

 extending beyond the luminous one." Several circumstances 

 concur in showing that there are influences also concerned 

 in the transmission of the photographic action which have 

 not yet been explained, as, for example, the influence which 

 the time of the day exercises on the rapidity with which 

 photographic impressions are made, the sun being much less 

 effective two hours after passing the meridian than two hours 

 before. There is also reason to suspect that the effect in 

 some way depends on the latitude, since a much longer time 

 is required to obtain an image under the bright skies of the 

 tropics than in England ; and it is even probable that there 

 is a difference in the sun's light in high and low latitudes, 

 because an image of the solar spectrum, obtained on a 

 Daguerreotype plate in Virginia, by Dr. Draper, differed from 

 a spectral image obtained by Mr. Hunt on a similar plate in 

 England. The inactive spaces discovered in the photographic 

 spectrum by M. E. Becquerel, similar to those in the luminous 

 spectrum, and coinciding with them, is also a phenomenon 

 of which no explanation has yet been given. Although 

 chemical action extends over the whole luminous spectrum, 



