sion
will be ruined by the quantity of mercury which would only improve a
light one.

If practicable, it is most expedient that the plate be submitted to the
action of mercury immediately on coming from the camera. I have
frequently, however, carried plates for miles in the plate-holders and
after exposing in the camera, brought them back to expose to mercury,
and obtained fair proofs; but for the reason before given, it is
advisable to carry along the bath, and bring out the impression on the
spot.

It is sometimes the practice of inexperienced operators to take the
plate off the bath and examine the impression by solar light. This
plan should be abandoned, as it is almost sure to produce a dense blue
film over the shadows.

This I am led to believe is occasioned by the action of light on the
yet sensitive portions of the plate, and made to appear only by
subsequent exposure to mercury, being equivalent to solarization.

There has been little said by our professors upon the subject of the
position of the plates while exposed to the mercurial vapour. Mr.
Hunt, in referring to this subject, says: "Daguerre himself laid much
stress upon the necessity of exposing the plate to the mercury at an
angle of about 45 deg.. This, perhaps, is the most convenient position
as it enables the operator to view the plate distinctly, and watch the
development of the design; but beyond this, I am satisfied there exists
no real necessity for angular position. Both horizontally and
vertically, I have often produced equally effective Daguerreotypes." I
presume from the last sentence of Mr. Hunt, that he has confined his
experiments to the smaller sized plates. Hence he may not have thought
of the effect of the vertical exposure of a large plate.

In America this is a subject of no little importance. When an
impression is to be developed upon a plate fifteen by seventeen inches,
were we to use an angle of about 45 deg., it would be found to make a
perceptible difference in the appearance of the image. By examining
the wood tops of our baths as formerly made, it will be found that
there is a great variation in the distance from the mercury to the
different portions of the plate. By measuring one of these tops for
the size plate above mentioned, I find the distance to the nearest
point between the mercury and the plate, to be thirteen, and the middle
point sixteen, and the furthest point twenty-one and a half inches: by
this we