platinum, carbontype, and the numerous other types which are springing
up in all directions for future consideration.

Now, in an ordinary pencil, pen and ink, or sepia sketch we have a
deposit of a dark, non-reflecting substance, which gives the outline of
a figure on a lighter background. The different gradations of shade
are acquired by a more or less deposit of lead, ink, or sepia. In
photography--at least in the ordinary silver process--the image is
formed by a deposition of metallic silver or organic oxide in a minute
state of division, either on glass, paper, or other suitable material.
This is brought about by the action of light and certain reagents. Light
has long been recognized as a motive power comparable with heat or
electricity. Its action upon the skin, fading of colors, and effect
on the growth of vegetable and animal organisms are well known; and,
although the exact molecular change in many instances is not clearly
understood, yet certain salts of silver, iron, the alkaline bichromates,
and some organic materials--as bitumen and gelatine--have been pretty
well worked out.

It is a remarkable and well-known fact that the chloride, iodide, and
bromide of silver--called "sensitive salts" in photography--are not
susceptible (at least only slowly) to change when exposed to the yellow,
orange, and red rays. The longer wave lengths of the spectrum, as you
know, form, with violet, indigo, blue, and green, white light. The
diagram on the wall shows this dispersion and separation of the
primitive colors. These--the yellow, orange, and red-- are called
technically "non actinic" rays, and the others in their order become
more actinic until the ultra violet is reached. The action of white
light, or rays, excluding yellow, orange, and red, has the effect of
converting silver chloride into a sub-chloride; it drives off one