92 Mr. Claudet on different Properties of Solar Radiation 



Having examined with a prism the light transmitted through 

 the glasses used in these experiments, I found that the red 

 absorbs two-thirds of the prismatic spectrum, from the space 

 covered by the green to the extremity of the violet, leaving 

 the red, orange, and a little yellow, followed by a very slight 

 trace of green. The orange glass gave more yellow, the green 

 being more decided. The light yellow glass intercepted the 

 half of the spectrum; the red was less intense than in the 

 preceding; the yellow occupied two-thirds of its total length, 

 and the green became very distinct ; but as far as my sight 

 allowed me to judge, I could not discover any portion of blue 

 in either case : certainly in the spectrum of the red glass there 

 was not the least trace of it. 



I will now detail the series of observations I have made 

 upon light transmitted through certain media — the vapours of 

 the atmosphere, and red, orange, and yellow glasses. These 

 experiments have brought forth some results which will 1 hope 

 contribute to lay the foundation of a more complete theory of 

 the photographic phaenomena. 



Having noticed, one densely foggy day, that the disc of the 

 sun was of a deep red colour, I directed my apparatus towards 

 it. After ten seconds of exposure I put the prepared plate in 

 the mercury box, and I obtained a round image perfectly 

 black. The sun had produced no photogenic effect. In an- 

 other experiment I left the plate operating for twenty minutes. 

 The sun had passed over a certain space of the plate, and 

 there resulted an image seven or eight times the sun's dia- 

 meter in length ; it was black throughout, so that it was evi- 

 dent, wherever the red disc of the sun had passed, not only 

 was there a want of photogenic action, but the red rays had 

 destroyed the effect produced previous to the sun's passage. 

 I repeated these experiments during several days successively, 

 operating with a sun of different tints of red and yellow. 

 These different tints produced nearly the same effect: where- 

 ever the sun had passed there existed a black band. 



I then operated in a different manner: not content with the 

 slow motion of the sun, I moved the camera obscura from 

 right to left, and vice versa, lowering it each time by means of 

 a screw. In this manner the sun passed rapidly over five or 

 six zones of the plate. Its passage was marked by long black 

 bands of the diameter of the sun, whilst the intervals were 

 white. It was then evident that the red and yellow rays, which 

 alone were capable of piercing the fog, had destroyed the 

 action produced by the little photogenic light which came 

 from the zenith. 



I then operated with coloured glasses. After exposing a 



