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light, or by the rays properly called pJwtogenie rays. One of the 

 first discoverers of this property was Dr. Draper of New York : 

 his experiments were made with the pure rays of the spectrum act- 

 ing on the Daguerreotype plate. Previously to this, however, Sir 

 J. Herschel had made similar observations on the action of the pure 

 rays of the spectrum on several kinds of photogenic paper. Dr. 

 Draper also found that the red, orange and yellow rays which 

 protect the plate from ordinary photogenic action, are themselves 

 capable, when isolated, of producing a peculiar photogenic effect. 

 In opposition to the hypothesis of an antagonistic or destroying 

 action exercised by the red, orange and yellow rays, M. E. Bec- 

 querel announced that those rays are endowed with the property of 

 continuing the action commenced by the photogenic rays. 



The author of the present paper has made a series of observations 

 on light transmitted through certain colouring media, through the 

 vapours of the atmosphere, and through red, orange and yellow 

 glasses. Having directed a camera obscura to the sun when his 

 disc appeared through a fog quite red, he obtained, after ten seconds, 

 a black image of the sun. The red sun had produced no photo- 

 genic effect, although the surrounding spaces had been sufficiently 

 affected by the photogenic rays coming from the zenith to attract 

 the white vapour of mercury; thus proving that the red rays have 

 no photogenic power. In another experiment he left the plate in 

 the camera during twenty minutes. The sun had passed over a 

 long space on the surface of the plate, and the result was a long 

 image of the sun, quite black throughout; so that not only the red 

 sun had produced no photogenic action, but the red rays had de- 

 stroyed the effect produced previous to their passage. Not content 

 with the result obtained by the slow motion of the sun, he next 

 moved the camera obscura from right to left, and vice versa, lower- 

 ing it each time by means of a screw. In this manner the sun was 

 made to pass rapidly over five or six zones of the plates, and its 

 passage was marked by long black bands, while the intervals were 

 white ; showing again that in order to destroy the action of the 

 photogenic rays, it was sufficient to cause the red rays to pass rapidly 

 over the spaces previously affected by the former. 



He afterwards operated with coloured glasses. After having 

 taken the impression of a piece of black lace by white light on a 

 Daguerreotype plate, he covered one half of the plate and exposed 

 the other to the radiation of a red glass. The mercury developed 

 an image of the lace on the part which had been acted on only by 

 the white light ; and the other part, which had afterwards received 

 the action of the red rays, remained black. The red glass had de- 

 stroyed the photogenic effect, precisely as was the case with the red 

 light of the sun. He made similar experiments with orange and 

 yellow glass, and obtained analogous results, but in different periods 

 of time. These experiments prove that the red, orange and yellow 

 rays destroy the effect of photogenic light, whether these rays are 

 produced by the prism or by the action of coloured media ; but the 

 author believes that he was the first to remark, that after the de- 



