204 PHOTOGRAPHIC PHENOMENA. SKCT. XXIV. 



marking 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 pho- 

 tographic 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 im- 

 pressions are made, the sun being much less effective 

 two hours after passing the meridian than two hours 

 before. There is also reason to Nsuspect 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 spec- 

 trum by M. E. Becquerel similar to those in the lumi- 

 nous spectrum, and coinciding with them, is also a phe- 

 nomenon of which no explanation has yet been given. 

 Although chemical action extends over the whole lumi- 

 nous spectrum and much beyond it in gradations of 

 more or less intensity, it is found by careful investiga- 

 tion to be by no means continuous ; numerous inactive 

 lines cross it coinciding with those in the luminous image 

 as far as it extends : besides, a very great number exist 

 in the portions that are obscure, and which overlap the 

 visible part. There are three extra-spectral lines be- 

 yond the red, and some strongly marked groups on the 

 obscure part beyond the violet ; but the whole number 

 of those inactive lines, especially in the dark spaces, is 

 so great that it is impossible to count them. 



Notwithstanding this coincidence in the inactive lines 

 of the two spectra, photographic energy is independent 

 of both light and heat, since it exerts the most powerful 

 influence in those rays where they are least, and also 

 in spaces where neither sensibly exist ; but the trans- 

 mission of the sun's light through colored media makes 

 that independence quite evident. Heat and light pass 



