modified by Coloured Glass Media, SfC. 93 



plate covered with a piece of black lace to daylight, I covered 

 one half, and submitted 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; the 

 other, which had afterwards received the action of the red 

 rays, remained black. The red glass had destroyed the pho- 

 togenic 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 analogous results, but in different times. 



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. 



AH 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 ob- 

 served 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, destroy this second effect by the 

 same coloured glass, and so on for many times, without 

 changing the properties of the surHice ; so that if we stop after 

 any of the exjiosures 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 mercurial 

 vapour. 



