t of the sesquioxide of uranium being in this case the hydrofluate, and
the time of exposure from the strength of the albumen negative fully an
hour of good sunshine. I have used for the solution of the uranic oxide
for this process a variety of acids with very similar results; the
sensitiveness of the prepared paper to light varying much, however. For
instance, a collodion negative with the hydrofluate paper producing a very
good print in half an hour of unsteady sun, while with a paper prepared
with the tartaric acid solution of the oxide, it gave an equally good
impression in less than five minutes of the same intermitting sunshine,
indicating thus a difference of sensitiveness of six to one in favor of
the tartrate."

"The rationale of this process is the reduction of the sesqui-oxide of
uranium, U2O3, on those parts of the paper exposed to the solar influence,
to a lower state of oxidation, the photo-oxide UO, the salts of which have
the property of forming with soluble alkaline ferridcyanides a rich
chocolate-brown precipitate, while the salts of the sesquioxide are
destitute of this reaction. Hence the brown deposit on the parts of the
picture on which the sun has been allowed to act when the developing
solution is applied, and the absence of any such appearance on those parts
which have been protected from its influence."

"As to the manipulatory details of this process, the paper is floated on
the solution in a dark room and hung up to dry, and then preserved from
light in a portfolio. If carefully secluded from light it appears to keep
well. After exposure for the proper time under this negative, there is in
some cases scarcely any visible impression; while in other cases,
particularly when using the tartaric solution, I have found the impression
very distinguishable, of a brownish or blackish shade, although still
quite faint. The development is best conducted by floating it, anything
like rubbing the picture being very objectionable."

"When the picture has fully come out, which is generally from three to ten
minutes at the very most, it is removed from the developing bath, placed
in cold water and washed very gently for a few minutes, the water being
frequently changed till it ceases to acquire a yellow tinge from the
dissolved red prussiate. The picture is then drained from the water,
pressed between folds of blotting paper, dried (I dry in the dark), and
the process is complete. * * * I may state,