258 MR. CLAUDET ON DIFFERENT PROPERTIES OF SOLAR RADIATION 
will serve for this restoration. I have obtained in this manner images equal in effect 
to those produced on plates prepared in the dark. 
This possibility of preparing plates in open day offers a great advantage to those 
who wish to take views or pictures abroad, and who cannot conveniently obtain a 
dark room. Again, in the case of a plate which has been left too long in the camera 
obscura, or accidentally exposed to the light, instead of rejecting it, we can restore 
its sensitiveness by placing it under a red glass. There is still another useful appli- 
cation of this property : if after one or two minutes’ exposure to the mercury we per- 
ceive the image is too rapidly developing, or presenting signs of solarization, which 
a practised eye discovers before it is too much advanced, we have only to stop this 
accumulation of mercury by exposing the plate for a few seconds to the red light, 
and again place it in the mercury box, to complete the modifications, which give the 
image all its tones and the most favourable tint. In truth, we may complete all the 
operations of the Daguerreotype in the open air, in the middle of a field if necessary. 
We can introduce the plate into the mercury box, in the same manner that we did in 
the camera obscura, by means of the same frame and red glass, which also serves 
to protect it when we take it from the mercury to rapidly view its development. I 
say rapidly, for if we expose it too long to the red light, the photogenic effect will be 
neutralized. We shall presently see that the time 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 necessary 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 yellow 
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 yellow 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 pho- 
togenic light. It is not even necessary that the restoration should be complete ; 
at each degree of restoration the plate is capable of receiving an accumulation 
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 yel- 
