208 



violet rays there exist luminous rays affecting the eyes with a sen- 

 sation, not of violet, or of any other of the recognised prismatic hues, 

 but of a colour which may be called lavender-grey, and exerting a 

 powerful deoxidating action. 



6. Chemical properties of the red end of the spectrum. The rays 

 occupying this part of the spectrum were found to exert an action of 

 an opposite nature to that of the blue, violet, and lavender rays. 

 When the red rays act on prepared paper in conjunction with the 

 diffused light of the sky, the discolorating influence of the latter is 

 suspended, and the paper remains white ; but if the paper has been 

 already discoloured by ordinary light, the red rays change its actual 

 colour to a bright red. 



7. The combined action of rays of different degrees of refrangibi- 

 lity is next investigated ; and the author inquires more particularly 

 into the effects of the combined action of a red ray with any other 

 single ray in the spectrum ; whether any, and what differences exist 

 between the joint, and the successive action of rays of any two dif- 

 ferent and definite refrangibilities ; and whether this action be ca- 

 pable, or not, of producing effects, which neither of them, acting alone, 

 would be competent to produce. The result was that, although the 

 previous action of the less refrangible rays does not appear to mo- 

 dify the subsequent effects produced by the more refrangible ; yet 

 the converse of this proposition does not obtain, and the simultaneous 

 action of both produces photographic effects very different from those 

 which either of them, acting separately, are capable of producing. 



8. In the next section, the chemical action of the solar spectrum 

 is traced much beyond the extreme red rays, and the red rays them- 

 selves are shown to exercise, under certain circumstances, a black- 

 ening or deoxidating power. 



9 . The author then enters into a speculation suggested by some in- 

 dications which seem to have been afforded of an absorptive action 

 in the sun's atmosphere ; of a difference in the chemical agencies 

 of those rays which issue from the central parts of his disc, and those 

 which, emanating from its borders, have undergone the absorptive 

 action of a much greater depth of his atmosphere ; and consequently 

 of the existence of an absorptive solar atmosphere extending beyond 

 the luminous one. 



10. An account is next given of the effect of the spectrum on cer- 

 tain vegetable colours, as determined b)'' a series of experiments, 

 which the author has commenced, but in which the unfavourable 

 state of the weather has, as yet, prevented him from making much 

 progress. 



1 1 . The whitening power of the several rays of the spectrum under 

 the influence of hydriodic salts, on paper variously prepared and 

 previously darkened by the action of solar light. The singular pro- 

 perty belonging to the hydriodate of potash of rendering darkened 

 photographic paper susceptible of being whitened by further expo- 

 sure to Hght is here analysed, and shown to afford a series of new 

 relations among the different parts of the spectrum, with respect to 

 their chemical actions. 



