124 THE PROGRESS OF PHOTOGRAPHY. 
The condition of these pensions was that the discovery 
should be given to the world. 
The success of Daguerre’s and Niepce’s efforts de- 
pended chiefly upon an accident. 
Daguerre used the same materials which had been 
used by Wedgwood and Davy, viz., chloride and nitrate 
of silver upon paper, with no better result. Niepce 
sometimes used metal plates coated with silver and em- 
ployed iodine to darken the exposed portions. Using 
vapor of iodine and silver coated plates. Daguerre 
found that the iodide of silver formed by exposing the 
plates to the vapor of iodine was sensitive to light, and 
that faint images of brightly illuminated objects became 
visible after an exposure of twoor three hours. Leaving 
one day what he supposed was an underexposed plate, 
in a dark cupboard, he was surprised on the following 
morning to find thereon a distinct and perfect image. He 
repeated the experiment again and again, always with 
the same result. 
By a process of elimination he ultimately discovered 
that the development of the image was due to the fact 
that the plates had been exposed while in the cupboard 
to the vapor escaping from a broken vessel of mercury. 
Daguerre at once utilized his fortunate discovery by 
placing his exposed plates over a dish of warm mercury, 
the vapor from which acted upon the iodized silver in 
exact proportion to the intensity of the light, by which 
each part of the surface of the plates had been affected. 
The result was a perfect picture, but now came again 
the problem which had confronted Davy. Some por- 
tions of the silver iodide had not been acted upon at all, 
and in a short time, exposure to the light must darken 
these and destroy the picture. It became necessary to 
remove from the plate all of the iodide of silver which 
had not been affected by the light. 
Daguerre at first used a strong solution of common 
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