J. F. Campbell Collection > Method of converting collodion negatives into positives, by heat

Seen through a microscope of stiong power,
by reflected light, the picture shines as frosted
silver, in points of coloured lighten a dark ground.
The points are nearest each other in the high
lights. By transmitted light the plate appears
covered with a fine dust, scattered thinly in the
shadows, more thickly placed in the half lights.
In the high lights the silver appears as a con-
tinuous film, with small holes in it at regular
intervals. Seen by both reflected and trans-
mitted light, the silver appears like a white
sand distributed on tlie glass, in several layers
in the white lights, but scattered in the shadows
and half lights
The possibility of producing photographic
pictures with the natural colours by some modi-
fication of this process, has frequently occurred
to me, and though I have hitherto failed to pro-
duce local colour, I would suggest that those who
have more leisure to devote to such experiments
should turn their attention to the subject. Many
substances may be made to apppear of any colour
by dividing them into plates sufficiently thin.
The brilliant colours which succeed each other
while the collodion is burning away, probably
depend on the thickness of the film through which
the silver dust is seen. The rate at which the
collodion burns must depend on the conducting
power of the substances in contact with it, and
the thickness of the film may depend on the
amount of silver deposited on various parts of
the plate. It may be that by some improvement
of this roasting process, the film maybe so affected
by the silver deposited in it, as to vary ui thick-
ness to the amount which will produce colour in
its proper position. I have once succeeded in
stopping the process when the sky of a land-
rcape was blue and the trees green, but that
sesult was accidental.