modified by Coloured Glass Media, ^c. 97 



Having exposed a plate with an engraving under the red 

 glass for sixty minutes, I replaced the red by a yellow glass, 

 without the engraving; after exposing the half of this plate 

 for five minutes under this yellow glass, the other half being 

 kept in the dark, the mercury produced a negative image on 

 the half exposed to yellow light, while the other gave no trace 

 of either positive or negative action. This result can only be 

 explained in the following manner: — 



First. That sixty seconds had not sufficed for the apparent 

 action of the red upon the half not exposed to the following 

 radiation of the yellow glass. 



Secondly. That nevertheless there had been the commence- 

 ment of an action upon which the yellow glass had to exercise 

 its destructive action. 



Thirdly. That while the yellow glass was occupied in de- 

 stroying the photogenic action of the red glass, restoring the 

 surface to its primitive state, it was exercising a photogenic 

 action upon the parts protected by the engraving from the red 

 rays, and in five minutes this photogenic action of the yellow 

 glass had produced a negative image by operating upon the 

 shadows of the drawing. 



It results from the experiments I have described, that the 

 solar radiation, when modified by coloured media, is in the 

 Daguerreotyj)e process endowed with several different photo- 

 genic actions, corresponding with various rays of the spec- 

 trum. 



The various photogenic actions of the modified solar radia- 

 tion have distinct characters; each of these modifications is 

 endowed with a photogenic power peculiar to itself, and which 

 gives an affinity for mercurial vapour to the Daguerreotype 

 plate. These various actions are so different, that we cannot 

 mix them artificially to assist each other, as they are antago- 

 nistic. The effect commenced by the blue rays is destroyed 

 by the retl and yellow; that which was produced by the red 

 is destroyed by the yellow ; the effect of the yellow rays is 

 destroyed by the red ; and the effect of the two latter is de- 

 stroyed by the blue; each radiation destroys the effect of the 

 others. Thus it appears that each radiation changes the state 

 of the surfiice, and each change produces the sensitiveness to 

 mercurial vapour when it does not exist, and destroys this 

 sensitiveness when it does exist. 



'J'he alteiiiate change of the state of the plate by these 

 various radiations seems to prove that the chemical compound 

 remains always the same under these diilcMent influences; 

 that there is no separation or disengagement of the constituent 

 elements. 



Phil. Mag. S. 3. Vol. 32. No. 2 1 3. Feb. 1848. H 



