96 Mr. Claudet on different Properties of Solar Radiation 



tnum of effect for the red glass. It is impossible to attribute 

 this effect to some feeble quantity of rays, properly called 

 photogenic, passing through the coloured glasses, for we have 

 seen that the blue and violet rays cannot operate under the 

 destructive action of the red rays ; this fact proves then evi- 

 dently, that if the red I'adiation has a photogenic effect, it 

 cannot be due to the same principle which produces the pho- 

 togenic efi'ect of the rays situated at the other extremity of the 

 spectrum. The yellow glass has also a peculiar photogenic 

 action of its own, it is a hundred times slower than that of 

 white light, whilst its destructive action is not more than ten 

 times as slow. We can obtain by the photogenic action of 

 the yellow glass an image almost identical, as to force and 

 colour, with an image produced by daylight; with this differ- 

 ence, that the excess of action does not give the blue solari- 

 zation which we observe upon plates strongly affected by day- 

 light. 



The different nature of the photogenic action of red, orange, 

 and yellow glasses, from that of the daylight, is also proved 

 by the fact, that the photogenic action produced by these co- 

 loured glasses cannot be destroyed by their own reversing 

 action, although the red will destroy the photogenic action of 

 the yellow, and both of these will destroy the action of day- 

 light. 



The double property of producing and destroying a pho- 

 togenic effect is manifested upon a specimen which offers on 

 one-half of the plate a negative image, and upon the other half 

 a positive image, produced at the same time by the same 

 radiation. The length of time necessary to operate with the 

 red glass has not allowed me to obtain a good impression, but 

 I have succeeded perfectly with the yellow glass. The expe- 

 riment is especially beautiful, and has been thus made: — 



I exposed one-half of the plate to daylight for one second, 

 keeping the other half in the dark. The entire plate was then 

 covered with an engraving and exposed under a light yellow 

 glass during ten seconds for the part previously affected by 

 white light, and during a hundred seconds for that which had 

 been kept in the dark. The yellow glass destroyed on the 

 first half the effect of the daylight wherever the plate was not 

 protected by the black lines of the engraving, and the parts 

 only which under these lines had been protected from the 

 destructive action, received the mercury, producing a negative 

 image ; while the same radiation of the yellow glass had ope- 

 rated photogenically upon the other half, developing a positive 

 image by the fixation of mercury upon the parts correspond- 

 ing to the lights of the engraving. 



