modified by Coloured Glass Media, 3fc. 89 



orange, and yellow rays were endowed with the property of 

 continuing the action commenced by the photogenic rays; 

 these latter he called exciting rays; to the first he gave. the 

 name of continuins. rays. 



M. Ed. Becquerel made his experiments on photogenic 

 papers, and added that he had observed the same effects on 

 the iodized silver plate. 



Dr. Draper of New York published in the Philosophical 

 Magazine for November 1842, some remarks on a class of 

 rays' which he supposed to exist in the light of the brilliant 

 sun of Virginia, and which had the property, when separated, 

 of entirely suspending the action of the diffused light from the 

 sky; these antagonistic rays extended from the blue to the 

 extremity of the red, and appeared to be almost as active in 

 preventing the decomposition of the iodide of silver as the 

 blue rays were in producing it. 



In January IBiS a memoir was read by me at the Society 

 of Arts, London, in a part of which 1 recommended opticians 

 to construct object-glasses in which they should particularly 

 correct the chromatic aberration of the long photogenic space 

 of the solar spectrum, even at the cost of the achromatism of 

 the less refrangible rays. This, however, had- been already 

 indicated, without my being aware of it at the time, by Sir J. 

 Herschel ; but I added that the greater separation of the 

 visual and photogenic focus which might result from such a 

 combination, according to the (luality of the glass employed, 

 would be an advantage', by dispersing, at the focus or on the 

 plate, beyond the photogenic lines, the red, orange or yellow 

 rays ; for the reason, that if they were brought to the same 

 point they would tend to neutralize and destroy the effect of 

 the photogenic rays. 



In October 1846, M. Lerebours announced to the Paris 

 Academy of Sciences that the red rays prevented the action 

 of the photogenic rays; this announcement induced Messrs. 

 Foucault and Fizeau to publish immediately similar results, 

 which they had previously consigned to the Academy in a 

 sealed memoir, bearing date May 1846. 



These communications of Messrs. Lerebours, Foucault and 

 Fizeau, led Dr. Draper to write a letter, published in the 

 Philosophical Magazine of February last, repeating his obser- 

 vations on the spectrum of Virginia, adding several other 

 analogous facts confirming the theory of a protecting and even 

 destroying action exercised by the least refrangible rays. Dr. 

 Draper, in the same letter, said that the rays which protect the 

 plate from ordinary photogenic action are themselves capable, 

 when isolated, of producing a peculiar photogenic effect. 



