silver nitrate solution. Pyrogallic is the most active agent, and might
be used alone with water; but for special reasons it is not desirable.
As a chemical it has a great affinity for oxygen, and will precipitate
silver from a solution containing, for instance, nitrate of silver. It
also combines with the metal, forming a pyrogallate--a dark brown, very
non-actinic material. The use of a few drops of AgNO_3 solution is very
evident. A deposit is added to the image already formed. Citric acid is
the retarder in this case. Alcohol is unnecessary, as the film is well
washed with water before the intensifier is used, consequently it flows
readily over the plate.

As regards fixing, or, more properly, clearing the image: it is the
simple act of dissolving out or from the film all free nitrate,
chloride, iodide, or bromide. Cyanide of potassium does not attack the
metallic deposit unless very strong. It has then a tendency to reduce
the detail in the shadows.

THOMAS H. MORTON, M.D.

* * * * *




GELATINE TRANSPARENCIES FOR THE LANTERN.

[Footnote: A communication to the Photographic Society of Ireland.]


Few of those who work with gelatine dry plates seem to be aware of the