120 THE PROGRESS OF PHOTOGRAPHY. 
The discoloring action of rays of light upon different 
substances has been the subject of observation for many 
centuries, but only within the last fifty years has experi- 
mental science demonstrated and explained the chemical 
reactions produced by light. 
The most ancient references to the subject may be sug- 
gested by the facts that inthe British Museum there is 
exhibited an object resembling a lens, which is said to 
have been found among the ruins of the ancient city of 
Nineveh ; and that Pliny recorded as a result of his ob- 
servation that yellow wax was bleached by exposure to 
sunlight. 
Fabricius, about the middle of the sixteenth century, 
searching for gold, found that the lunar cornea, or horn- 
silver, could be prepared by addinga solution of common 
salt to a solution of silver nitrate, and he was surprised 
at, and recorded the fact that this white compound 
turned quickly black when exposed to the sunlight. 
No thought of utilizing this discovery seems to have 
occurred to him. 
In 1727, Schultze obtained copies of writing, by writing 
first upon a paper whose surface had been prepared 
by chalk and nitrate of silver. It was found that the 
sun’s rays passing through comparatively transparent 
paper blackened the surface of the prepared paper be- 
neath, except where intercepted by the ink forming the 
letters, producing a white copy upon a black background. 
But it does not appear that Schultze made any attempt 
to fix the result of his experiment, or even put it to any 
practical use, and the experiment stands as an isolated 
one, yielding no fruit for half a century at least. 
It remained for the distinguished chemist, Scheele, of 
Straslund, to first investigate and, in a measure, demon- 
strate, the chemical effect of the spectrum, thus stimu- 
lating the long line of investigation which led to the dis- 
covery of photography. 
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