LXXXII
image of a person is formed in the proper position on the glass.
Before the portrait is taken a glass plate is coated with a thin
layer of Collodion and immersed in a solution of nitrate of silver
in a dark room, and the plate is inserted in the camera obscura.
The action of light upon this Iodide of silver causes a decom
position, in virtue of which those parts exposed most to the
light are dark and vice versâ. This portrait is called the
negative. To render the plate insensitive to light and to pro
tect it from injury, those parts of Iodide of silver, on
which light has not yet acted, are removed and the plate is dried
up. For producing the positive picture the plate is laid on a
sheet of paper impregnated with Iodide of silver and the whole
is exposed to the action of light for a certain time. The light
parts of the negative being most affected and the dark parts
least so, a copy will be obtained on which the lights of negative
