At a meeting of the British Association, Professor Grove described a
process by which positive calotype pictures could be directly obtained;
and thus the necessity to transfer by which the imperfections of the
paper are shown, and which is moreover a troublesome and tedious
process, is avoided. As light favors most chemical actions, Mr. Grove
was led to believe that a paper darkened by the sun (which darkening is
supposed to result from the precipitation of silver) might be bleached
by using a solvent which would not attack the silver in the dark, but
would do so in the light. The plan found to be the most successful is
as follows: ordinary calotype paper is darkened till it assumes a deep
brown color, almost amounting to black; it is then redipped into the
ordinary solution of iodide of potassium, and dried. When required for
use it is drawn over dilute nitric acid--one part acid to two and a
half parts water. In this state, those parts exposed to the light are
rapidly bleached, while the parts not exposed remain unchanged. It is
fixed by washing in water, and subsequently in hyposulphite of soda, or
bromide of potassium.
Mr. Grove also describes a process for converting a negative calotype
into a positive one, which promises, when carried out, to be of great
utility.
Let an ordinary calotype image or portrait be taken in the camera, and
developed by gallic acid; then drawn over iodide of potassium and
dilute nitric acid and exposed to full sunshine; while bleaching the
dark parts, the light is redarkening the newly precipitated iodide in
the lighter portions and thus the negative picture is converted into a
positive one.
The calotype process has been applied to the art of printing, in
England, but it possesses no advantages whatever over the method, with
type, now so gloriously brought to perfection; and I can hardly think
it will ever be made of any utility. For the benefit of the curious,
however, I will give Mr. Talbot's method.
Some pages of letter-press are taken printed on one side only; and
waxed, to render them more transparent; the letters are then cut out
and sorted. To compose a new page lines are ruled on a sheet of white
paper, and the words are formed by fixing the seperate letters in their
proper order. The page being ready, a negative photograph is produced
from it, from which the requisite number of positive photogenic copies
may be obtained.
Another method, which requires the use of the camera, consists in
employing large letters painted on rectangular pieces of wood, colored
white. These are arranged in lines on a tablet or board, by slipping
them into grooves which keep them steady and upright, thus forming a
page on an enlarged scale. It is now placed before a camera, and a
reduced image of it of the required size is thrown upon the sensitive
paper. The adjustments must be kept invariable, so that the
consecutive pages may not vary from one another in the size of the
type. Mr. Talbot has patented his process, but what benefit he expects
to derive from it, I am at a loss to determine.
Enlarged copies of calotype or Daguerreotype portraits may be obtained
by throwing magnified images of them, by means of lenses, upon calotype
paper.