see that one point of the plate is eight and a half inches
further from the mercury than the nearest point; even this is not the
variation there would necessarily be, were we to adopt the angle of 45
deg. as urged by Daguerre.

Among our principal professors, the bevel top will not be found in use
where the large plates are used. Should any one feel desirous to test
more minutely the effect produced by a bevel top bath, I would suggest
to them to place a frame, so constructed as to hold three sixth size
plates, and fit it to the top of the bath, and so arrange it with
openings that the plates may be placed, one at the nearest point of the
mercury, the second midway, and the third to the greatest distance, and
by placing the plates over at one and the same time, the experimenter
will be enabled to judge if there exists a difference in the
developing. In speaking of the above, reference is had to baths to the
ordinary heights used by operators.

We will now proceed to examine the effect produced by mercurial vapor
upon the plate at different lengths of exposure. In some
investigations which I have made upon the appearance of the
Daguerreotype impressions when developed over mercury at 90 deg. C.
(194 deg. F.), the following was the result. Plates, coated and
exposed to light in our usual manner of operating, produced on exposure
of

1/2 minute, whole impression, deep blue.

1 minute, ashy and flat; no shadows; linen, deep blue.

1 1/2 minute, coarse and spongy; shadows, muddy; drapery, dirty reddish
brown.

2 minutes, shallow or watery; shadows, yellowish; drapery, brown.

2 1/4 minutes, soft; face, scarcely white; shadows, neutral; drapery,
fine dark brown linen somewhat blue.

2 1/2 minutes, clear and pearly; shadows, clear and positive, of a
purple tint; drapery, jet black, with the dark shades slightly frosted
with mercury.

2 3/4 to 3 minutes, hard and chalky; shadows, harsh; drapery,
roughened, and misty with excess of mercury.

The foregoing results will be found general.

There are numerous opinions among our operators in regard to the
quantity of mercury necessary for a bath. As regards this, I need only
say, similar results occur when two pounds or two ounces are used, but
the quantity generally employed is about a quarter of a pound. I am of
the opinion that one ounce will answer as well as a larger quantity. I
know of no better proof in favor of a small quantity than that
presented in th