The tank is constructed as follows: Procure two pieces of best white
plate glass, about 6 inches square; between these place a piece of
rubber of the same size square, and about 3/8 of an inch thick. In the
center of this rubber cut out a circle about 4 inches diameter, and
from one of the corners to the center of the circle cut out a narrow
strip 1/4 inch wide; this serves as the mouth of the tank. The two
pieces of glass and the rubber are cemented together with rubber
cement; then, to hold it firmly together, two brass flanges are used
as a clamp, with four screws at an equal distance apart; a thin sheet
of rubber is on the glass side of the flanges to prevent direct
contact with the glass, the center remaining clear for the rays of
light to pass through solution and glass.
One of the best orthochromatic effects made through this tank is with
a three-grains-to-the-ounce solution of bichromatic of ammonia or
bichromate of potassium. In this method there is no preparation used
on the plate. A common rapid dry plate is exposed through this
solution; the exposure, however, is about twenty times longer than it
would be if you removed the tank with the yellow solution, or, in
other words, if a dry-plate is exposed one minute without the yellow
solution it would have to be exposed twenty minutes through a
three-grain solution of bichromate of potassium or ammonia.
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