94- Mr. Claudet on different Properties of Solar Radiation 



Having exposed a plate to the two actions alternatel}', first, 

 once upon one zone, twice upon anotlier, 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 

 exposing 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 ^^ith 

 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 sensi- 

 tiveness. 



This restoring property of the coloured glasses may be of 

 great use in the Daguerreotype 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, 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 application 

 of this property: if after one or two minutes' exposure to the 

 mercury we perceive the image is too rapidly developing, or 

 presenting signs of solarization, which a practised eye disco- 

 vers 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 com- 

 plete 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 



