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 see that one point of the plate is eight and a half
inches further from the mercury than the nearest point;