EDWARD ELLSWORTH. 2 
He discovered the facts, in 1777, that the different col- 
ored rays of light exercised an unequal effect upon the 
salts of silver ; that silver chloride was darkened by the 
violet or blue rays in much less time than by the yellow 
or red rays. He also first discovered the cause of the 
darkening, viz.: that the effect of white light upon chlo- 
ride of silver is to decompose it. This he accomplished 
by exposing chloride of silver under water, pouring off 
the water, and then adding a little nitrate of silver, 
whereby chloride of silver was again produced ; proving 
that a decomposition of the former substance had taken 
place, and that chlorine had been dissolved in the water. 
The conclusions of Scheele were disputed by some, 
even the distinguished Count Rumford maintaining that 
the active agent was heat, and not light. But the main 
results of Scheele’s experiments are not disputed to-day. 
Still no one appeared to conceive any practical use to 
which the discovery of Scheele could be put. Although 
for more than a century suggestions had appeared along 
the way, they have waited until almost the last half of 
the nineteenth century to be developed into practical 
media for utilizing the knowledge of the philosophers 
that sunlight will print an image upon a surface coated 
with the salts of silver. 
The camera had already been invented, and, by means 
of a lens, forms of which had been constructed ages _ be- 
fore, beautiful pictures of scenery had been reflected 
upon the white walls of a chamber ; and the secret door 
of photography seems almost to have been unlocked by 
a Frenchman, de la Roche, in 1760, in a book called ‘‘ Gi- 
phantie,’’ wherein he transports his hero to a strange 
land, where he is shown the method by which the native 
genii produce pictures. ‘‘ You know,” he writes, ‘‘ that 
rays of light reflected from different bodies form pic- 
tures, paint the image reflected on all polished surfaces ; 
for example, on the retina of the eye, on water and on 
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