In using different colored solutions, collodion, etc., I have found that one will change the focus and the other not.  With some screens you must focus with them in their positions; take away the screen, and the picture appears out of focus.  I cannot fully explain why it is, and for this reason will not make the attempt; experience alone can teach it.

Another thing that has been tried lately is to do away with the yellow screen by substituting a yellow coating direct on the plate.  No doubt the focus on an object that requires absolute sharpness is somewhat affected by the use of a glass.  We have been successful, on a small scale, to coat the plate with the following yellow solution: 

Place in a tray enough of a saturated solution of tropaeolin in wood alcohol to cover the plate; allow it to remain ten seconds.  It is necessary that the plate should be bathed previously in erythrosine and dried.  Before applying the tropaeolin, which, being in alcohol, dries in a few minutes, have some blotting paper on hand, as the solution gathers in a pool and leaves bad marks on the end of the plate.

The plate can be developed in the usual way.  Try it and see the results.—­Reported in the Beacon.

* * * * *

PLATINOTYPE PRINTING.[1]

   [Footnote 1:  A communication to the North London Photographic
   Society.]

Platinotype, which may be considered to be the most artistic of photographic printing processes, may be separated into its three modifications—­the hot bath and cold bath, in which a faintly visible image is developed, and the Pizzighelli printing-out paper.  The hot bath process, again, may be divided into the black and white and sepia papers.  I intend to give you a rough outline of the preparation of the paper and working of these modifications, concluding by demonstrating the hot bath method, and handing around prints by it.

Platinotype may almost be styled an iron printing process, for, while no trace of iron or its salts is found in the finished print, certain salts of iron are mixed with the platinum salt, which is platinum combined with two atoms of chlorine (PtCl2), as a means for readily reducing it; this, however, cannot be effected without the presence of neutral oxalate of potash, hence the use of the oxalate bath.  There is no platinum in the paper for the cold bath process, it being coated with ferric oxalate mixed with a very small quantity of chloride of mercury—­somewhere about one grain to an ounce of ferric oxalate solution.  When dry it is ready for exposure, which is about three times less than with silver printing.