256 
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 I hope con- 
tribute to lay the foundation of a more complete theory of the photographic pheno- 
mena. 
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 another 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 diameter in length: 
it was black throughout, so that it was evident, 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 expe- 
riments during several days successively, operating with a sun of different tints of 
red and yellow. These different tints produced nearly the same effect : wherever 
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 pro- 
duced by the little photogenic light which came from the zenith. 
I then operated with coloured glasses. After exposing a plate covered with a piece 
of black lace to daylight, I covered one half, and submitted the other to the radia- 
tion 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 ; the other, which had afterwards received 
the action of the red rays, remained black. The red glass had destroyed the photo- 
genic effect in the same manner as it had been done by the red light of the sun. 
I made the same experiment with orange and yellow glasses, and obtained analo- 
gous results, but in different times. 
