408 CHEMICAL FOKCE IN THE SPECTRUM. [MEMOIR XXIX. 



action of diffused light on this iodide is mainly due to 

 the more refrangible rays it contains, we are brought by 

 these experiments to the following conclusions : 



1st. Every ray in the spectrum acts on silver iodide. 



2d. The more refrangible rays apparently promote the 

 action of daylight on that substance ; the less refrangible 

 apparently arrest it. 



3d. For the display of this arresting or antagonizing 

 effect, it is not necessary that the less and more refran- 

 gible rays should be acting simultaneously. An interval 

 may elapse, and they may act successively. Hence the 

 effect is not due to the contemporaneous interference of 

 waves of different periods of vibration with one another 

 the material particles of the changing substance of 

 the silver iodide are involved. 



I abstain for the moment from giving further details 

 of these spectrum impressions. That has been very 

 completely done by Herschel in the case of one I sent 

 him many years ago. His examination of it, illustrated 

 by a lithograph, may be found in the Philosophical Mag- 

 azine (Feb., 1843). I shall have to return to the subject 

 of the behavior of silver iodide in presence of radiations 

 on a subsequent page of this Memoir. 



The main point at present established is this, that the 

 iodide under proper treatment is affected by every ray 

 that a flint-glass prism can transmit, and therefore it is 

 altogether erroneous to suppose that chemical force is 

 restricted to the more refrangible portions of the spec- 

 trum. 



2d. Case of Bitumens and Hesins. 



These substances are of special interest in the history 

 of photography, since in the hands of Niepce they prob- 

 ably were the first on which impressions in the camera 

 were obtained and fixed. Their use has been abandoned 

 in consequence, as it seems to me, of an incorrect opinion 



