that iodine is almost insoluble in water, requiring seven thousand parts
of water to dissolve one of iodine, or one grain to a gallon of water.
Yet its affinities for silver and other substances are so powerful as to
prevent its existing in an insulated state, hence we can account for the
frequent occurrence of a plate presenting parts of an image over its surface.
It is quite evident that those parts of plate's surface covered with moisture
are nothing like as sensitive to the iodine as those parts perfectly free.

Exposure of the plate in the Camera, and Position.--The time of
exposure necessary to produce an image upon the Daguerreotype plate,
can only be determined by experiment, and requires a liberality
of judgment to be exercised on the part of the operator.
The constant variation of the light renders it impossible to lay
down any exact rule upon this point. Light is not alone to
be considered; the amount of coating exercises a deviating influence,
also the subjects to be represented are not equally photogenic,
some requiring much longer time of exposure than others.
This may be easily observed by exposing the plate at the same
time to a plaster bust and a piece of black velvet, the first
being a much stronger reflector of light than the latter:
the time necessary to produce a well developed image
of the velvet being about six times longer than that
required to produce an equally defined image of plaster.
The manner of judging correctly of the time is by the appearance
of impression after it has been developed by the mercurial vapors.
Should it present a deep blue or black appearance it is solarized
or over-timed. This sometimes is to an extent, that a perfect
negative is formed, the white being represented black,
and the dark light.