monium nitrate, a mere bye-product which is removed by passing
into solution. This plate is used wet—just after sensitizing—and
is the kind generally employed for photo-engraving.
Dry plates, such as are commonly used for ordinary photo
graphic purposes, are prepared by coating glass plates with a
gelatin emulsion of sensitive silver salts. These plates are used
dry and at any time.
If we should take a sensitized plate and expose it to actinic
rays a change would be wrought in the sensitive mass; and, if the
whole plate be evenly exposed, the change wrought will be uni
form, but, if different parts be unequally exposed, the change
wrought will be proportional to the exposure and to the intensity
of the action. If, then, such a plate be exposed in a camera,
properly focussed upon some object, the rays of light, varying in
intensity with the different parts from which they are reflected,
will affect the plate with different intensities, causing an image to
be formed upon it. The action leaves an invisible result. To
bring out the image it is necessary to " develop" it.
Development is effected by the action of such reducing sub
stances as pyrogallic acid or ferrous sulphate. These compounds
act upon the sensitive film wherever it has been affected by light,
separating metallic silver as a dark powder in a very finely dit.
vided state. Those portions of the plate which were exposed to
light reflected from white objects or high lights appear first,
then those from half tones and, finally, the detail in the shadows.
Further, as pure white light affects the sensitive film strongest
we will have the densest precipitation of metallic silver at those
parts of the negative corresponding to the high lights, gradually
diminishing, through the various shades, until at those parts cor
responding to blacks the negative will be found unaffected. This
then gives us a picture in black and white but, in reverse of the
object photographed. When the process of development has pro
ceeded sufficiently to render details visible the plate is washed
and then placed in a " fixing " bath.
The object of this bath is to completely remove 'all the unre
duced salts, i. e., those which were unaffected by the light and
subsequently reduced by the developer. This is effected by a
solution of potassium cyanide or hyposulphite of soda; the action
is a chemical one, the insoluble sensitive salt of silver being con
verted into a soluble salt corresponding to that of the fixing bath,