SECT. XXIV. IMAGES OF THE SUN. 213 



of this very singular phenomenon in a real difference between the 

 chemical agencies of those rays which issue from the central por- 

 tion 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 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." M. Arago observed that the rays from the centre of the 

 sun have a greater photographic power than those from the edges, 

 and the photographic images of the sun, taken on glass by M. 

 Niepce, were blood-red, much deeper in the centre, and on one 

 occasion the image was surrounded by an auriol. Several cir- 

 cumstances 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 photo- 

 graphic 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 spec- 

 trum, 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 ; pos- 

 sibly the chemical rays may be absorbed by the atmosphere with 

 those of light. Although chemical action extends over the whole 

 luminous spectrum, and much beyond it, in gradations of more 

 or less intensity, it is found by careful investigation 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 extraspectral 

 lines beyond the red, and some strongly marked groups on the 



