To produce a good negative a collodion is necessary, the film of
which is more permeable than that for positives, so that a heavier
deposit of iodide of silver may be carried in the film, to the end
that in the development a denser deposit of metallic silver may be
secured on all parts of the image, but especially on the high
lights, which should be sufficiently dense to prevent the
transmission of light almost altogether, or at least to exclude the
light sufficiently to allow the shadows and intermediate shades to
print to the proper depth, while the high lights of the face and
such parts of the clothing as are white shall be only slightly
discolored.
There are very many formulæ for such collodions, some of which have
proved to be eminently satisfactory, and among the best are classed
the following: