equivalent of chlorine. Thus, silver chloride, Ag_2Cl_2=Ag_2Cl+Cl.
When water is present the water is decomposed. Hydrochloric acid, HCl,
hypochlorous acid, HClO is formed.

The iodide of silver in like manner is changed into a sub-iodide; but
with water hydriodic acid is formed unless an iodine absorbent be
present--then into hypoiodic acid. The silver bromide undergoes
a similar change. When with light alone, a sub-bromide,
Ag_2Br_2=Ag_2Br+Br, and with water hypobromous acid. It is important
to bear this in mind, as one or other, and frequently both iodide and
bromide of silver, is the sensitive salt requisite or used in producing
the invisible image.

The theory regarding these sensitive salts of silver is that, being very
unstable, _i. e._, ready to undergo a molecular change, the undulations
produced in the ether, which pervades all space, and the potential
action or moving power of light is sufficient to disturb their normal
chemical composition; it liberates some of the chlorine, iodine, or
bromine, as the case may be. This action, of course, applies to light
from any source--the sun, electricity, or the brighter hydrocarbons,
also flame from gas or candle, whether it comes direct as rays of white
light or is reflected from an object and conducted through a lens as a
distinct image upon the screen of a camera.

I have no time to speak on the subject of lenses, only just to mention
that they are, or ought to be, achromatic, so as to transmit white light
and of perfect definition, and the amount of light passed through should
be as much as possible consistent with a sharp image--at least when
rapid exposure is attempted.