MODIFIED BY COLOURED GLASS MEDIA, ETC. 
257 
Then, having exposed a plate to daylight, I subsequently covered it with a piece 
of black lace, and exposed it again under a red glass : this produced a negative image. 
The red had destroyed the effect of the white light in the intervals of the lace, the 
threads of which preventing the action of the red glass, produced a white image upon 
a black ground. In operating in this manner upon one-half of the plate, exposing the 
other half covered only by the same lace to the light of the day, I obtained by the first 
a negative, and by the second a positive image. The orange and yellow glasses give 
the same result, paying regard to the difference of time in their respective actions. 
All these experiments prove what has been already observed by others before me, 
but in a different manner, that the red, orange and yellow rays destroy the effect of 
the photogenic light, whether these rays be produced by the prism or by the action of 
coloured media ; but, I believe, it has not been observed by any one before me, that 
after the destruction of the photogenic effect the plate is perfectly restored to its 
former sensitiveness to white light. 
After exposing a plate to daylight, and then submitting it to the destructive 
action of red, orange or yellow rays, it will be found again sensitive to the same white 
light. 
I have obtained plates which present an equal and uniform image, although the 
one-half had been exposed to light, and then restored by the red, orange or yellow 
glass, while the other half had received only the single and final radiation. We 
may then expose a plate to light, destroy this effect by the action of red or 
yellow glass, which renders it again sensitive ; then expose it again to light, de- 
stroy this second effect by the same coloured glass, and so on for many times, 
without changing the properties of the surface ; so that if we stop after any of the 
exposures to white light, the plate will receive mercury ; but if we stop after any of 
the exposures to red, orange or yellow light, we shall obtain no fixation of mercu- 
rial vapour. 
Having exposed a plate to the two actions alternately, first, once upon one zone, 
twice upon another, and so on until the last zone had been exposed and destroyed 
six times, I covered the plate with a piece of black lace or an engraving, finally ex- 
posing the whole to white light ; the result was an equal deposit of mercury upon 
the whole surface of the plate. The impression of the lace or engraving seemed to 
be the result of a single exposition to light, as would have been the case with a normal 
plate; therefore the action of the red, orange, or yellow glass upon a plate previously 
affected by light, produces the same effect as a fresh exposure to the vapours of 
iodine or bromine, when we wish to restore the plate to its first sensitiveness. 
This restoring property of the coloured glasses may be of great use in the Da- 
guerreotype manipulation. Instead of preparing the plates in the dark, it may be 
done with impunity in the open light. To give sensitiveness, we have only to place 
the plate for some minutes under a red glass before putting it in the camera obscura. 
The frame or box used to hold the plate, if furnished with a red glass at the bottom, 
2 L 
MDCCCXLVII. 
