MODIFIED BY COLOURED GLASS MEDIA, ETC. 
259 
low rays were the same as that of the more refrangible rays, it could never develope 
itself under the destructive action which the same glasses carry with them. 
But there is yet more; each ray of the spectrum has its own photogenic action, 
and they are in this respect independent of each other, and of a different kind ; so 
that the one cannot continue the effect commenced by the other, whether it be for 
the production or for the destruction of the photogenic effect. I would again ob- 
serve, whenever I speak of a photogenic effect, I mean that which gives to the Da- 
guerreotype plate the property of attracting the vapours of mercury. 
If we expose a plate covered by an engraving to the red light 5000 times longer 
than is required to produce an effect by white light, we obtain by the fixation of 
mercury a feeble image, the lights of which are of a grey tone. I could never go 
beyond this feeble image, which appeared to be the maximum of effect for the red 
glass. It is impossible to attribute this effect to some feeble quantity of rays, pro- 
perly 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 evidently, that if the red radiation has a photogenic effect, it 
cannot be due to the same principle which produces the photogenic effect of the rays 
situated at the other extremity of the spectrum. The yellow glass has also a pecu- 
liar 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 ob- 
tain 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 difference, that the 
excess of action does not give the blue solarization which we observe upon plates 
strongly affected by daylight. 
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 pro- 
duced by these coloured 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 daylight. 
The double property of producing and destroying a photogenic 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 
experiment 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 
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