what is termed a "safe edge" upon which should rest the tissue in
order to prevent the margin from being insolubilized by the reductive
action of light. If this precaution were neglected it would be impossible
to strip off the paper without tearing the proof when the tissue is
applied on the support upon which the image is to be developed.

Before exposing it is advisable to ascertain what the printing qualities
of the negative are by making on silvered paper a proof of it--_not
over-printed--_and another of the photometer, both being exposed at the
same time and for the same period. This done, compare the proof from the
negative cliche with those of the photometer, and mark the negative with
the number of that of the photometer to which it corresponds, stating the
shade of the proof next to it; for example: _No. 2_; _No. 3 faint, or
commences to appear,_ etc. This No. 2 and the observation will indicate
the intensity of the negative and serve as a guide for printing on the
tissue, since, as before explained, the silver paper is practically of the
same sensitiveness as the tissue prepared for negatives of the ordinary
intensity.

_Exposure.--_To print, the tissue is laid over the negative, taking care
that it covers the safe edge, and a strip of silvered paper placed in the
photometer, then both the printing frame and the photometer are exposed to
light side by side.

Unless the negative be weak, when more vigor is obtained by exposing in
sunshine, the printing should be done in the shade. It is a well-known
fact that the part of the bichromated film corresponding to the half
tones in the lights are not sufficiently impressed in comparison to the
blacks while impressed in direct sun's light in this as well as in the
collotype, photogravure and other processes with the chromic salts,
because the luminous action through the bare glass, or nearly so, which in
the negative represent the shadows and half blacks, is more energetic in
proportion than through the other parts, from which it results that these
parts being most acted on are made deeply insoluble through the thickness
of the film, and then require to be cleared by a treatment with water at a
higher temperature than the parts representing the half tints in the
lights of the picture, which are but superficially and slightly insoluble,
can stand.

From time to time during the exposure the print in the photometer is
examined, and when a certain picture is print