modified hy Coloured Glass Media, Sfc. 95 



required to observe the state of the image is not sufficient to 

 affect its affinity for mercury, if it be found requisite to replace 

 it in the mercury box. The exposure under red glass neces- 

 sary to destroy the effect produced by white light, must be a 

 hundred times longer than has been the exposure to white 

 light, that of the orange glass fifty times, and that of the yel- 

 low glass only ten times; thus a plate exposed to white light 

 for a second will be restored to its former sensitiveness in ten 

 seconds by the vellow glass, in fifty by the orange, and in a 

 hundred by the' red. As soon as the sensitiveness of the plate 

 affected by white light is restored by the coloured glasses, it 

 may be affected again by the photogenic light. It is not even 

 necessary that the restoration should he complete ; at each 

 degree of restoration the plate is capable of receiving an accu- 

 mulation of photogenic effect. If the red rays have not acted 

 more than fifty times longer than the daylight, only half of the 

 effect will be destroyed; if twenty-five times longer, one-fourth; 

 and so on in proportion. 



Besides the destructive action of the red, orange and yellow 

 glasses, these same radiations are endowed with a photogenic 

 power, that is to say, they have, like the blue and violet rays, 

 the power of causing the fixation of mercurial vapour. There- 

 fore these radiations are endowed with two contrary actions ; 

 the one destructive of the effect of the photogenic light, and 

 the other analogous to the effect of this light. 



If the red, orange, and yellow radiations of the prism had 

 not also the power of operating photogenically, it might be 

 supposed that this action of the coloured glasses was due to 

 some of the most refrangible rays transmitted by these coloured 

 media. But this cannot be; for if the photogenic action of 

 the red, orange, and yellow 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 indepen- 

 dent 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 photo- 

 genic effect. I would again observe, whenever I speak of a 

 photogenic effect, 1 mean that which gives to the Daguerreo- 

 type plate the property of attracting the vapours of mercury. 

 If we expose a plate covered by an engraving to the red 

 li<dit 5000 limes longer than is required to produce an effect 

 by white light, we obtain by the fixation of mercury a feeble 

 image, the iiglits of which are of a gray tone. I could never 

 go beyond this feeble image, wliich appeared to be the maxi- 



