IMPROVEMENTS IN THE PRODUCTION OF PHOTOGRAPHIC PICTURES
Year: 1857

page_text: PROVISIONAL SPECIFICATION left by William McCraw at the Office
of the Commissioners of Patents, with his Petition, on the 2nd July
1857.
I, WILLIAM MºCRAW, of Edinburgh, in the County of Mid-Lothian, North
Britain, Artist, do hereby declare the nature of the said Invention for
"IMPROVEMENTS IN THE PRODUCTION OF PHOTOGRAPHIC PICTURES," to be as follows,
that is to say:-
This Invention relates to certain improved processes of producing positive
photographic images or pictures on white or light-tinted substances, either
vitreous, animal, or vegetable.
According to one modification of the improved processes, the operator
proceeds by first coating a slab of porcelain (by preference unglazed) with
suitable photographic collodion or other suitable medium. The slab so coated
is then excited in the nitrate of silver bath in the usual way, and is afterwards
exposed in the focus of the camera. The slab so treated is then taken into
a darkened chamber, where the prepared surface is saturated with a weak
solution of protosulphate of iron or pyrogallic acid or other developing solution,
and it is then immediately washed before any visible or appreciable effect is
produced. The slab is now momentarily exposed to the action of subdued
daylight or to an artificial light, and is immediately afterwards treated second-

page_text: arily with a developing fluid, when the latent image appears as a positive
photograph, with the lights and shades correct as in nature, but with the
right and left reversed, as is usual with daguerrotypes. To obviate this
objectionable feature a negative photograph on glass is taken, and placed in
front of the camera at a suitable distance off, with a mirror or reflector placed
behind the image at an angle of about 45 degrees, to act upon the principle
of the microscopic reflector. The effect of this is that on focussing the camera
a clear and well-defined image of the kind required is obtained. The porcelain
or other prepared tablet is now placed in the focus of the camera, and the
resultant image is developed in the darkened chamber as usual, and fixed
with a solution of cyanide of potassium or of hyposulphite of soda, the image
surface being finally washed in the ordinary manner. The effect may be
heightened at this stage by the application of a solution of chloride of gold
of the strength of one grain of gold to one ounce of water. Various effects
may be produced by varying the strength of the solution of gold. The picture
is now dried at a fire, being subjected to the action of a considerable but
gradual heat, and when the slab or plate is cool the image is coated with
varnish. Colour may be applied to the image either with or without, or
before or after varnishing, by any of the known modes of colouring photographs.
By pursuing this general routine, photographic pictures may be produced
upon porcelain, china, and other kinds of earthenware slabs, and on white or
opal glass, also on ivory or bone, and on prepared wood, or on any white or
light-coloured enamels; photographs of this kind are much more enduring
than those on paper. The new process is valuable not only for portraiture
purposes but also for the embellishment of various classes of manufactures
and fancy articles. It also affords the means of producing positive transparent
photographic slides for the stereoscope, from binocular negatives with collodion
or other media, either on white or opal glass, or on the ordinary transparent
glass. If transparent glass is used, the picture surface of the negative image
is turned towards the mirror or reflecting surface, the reverse side being
towards the lens. Pictures so taken will then require no front protecting
glass, as the image is correctly given when viewed through the actual glass.
on which each is taken, whilst the picture side is protected by a ground glass,
placed behind in the usual manner. If white or opal glass is used, either
side of the negative may be turned towards the lens, a transparent protective
glass being placed before or at the back of the resulting picture, as may be
required.
The chief essential features of the present Invention are, the obtainment
of direct positions in the camera of the kind necessary to be viewed as positives

page_text: on white surfaces, the processes having no reference to photographs on paper,
which the present improvements are intended to supersede.
The adaptation or arrangement of the mirror or reflector for obtaining a
brilliant and sharp image from negative photographs by the aid of the camera,
and the producing by both or by either of these systems positive photographs
on porcelain, china, ivory, bone, mother-o'-pearl, white enamel, white or opal
glass, and other surfaces, which by reason of the inequalities of their surfaces,
or the rigid nature of the materials, are rendered unfit for receiving photo-
graphic images from negative photographs in the ordinary way.