We use Optical Character Recognition (OCR) during our scanning and processing workflow to make the content of each page searchable. You can view the automatically generated text below as well as copy and paste individual pieces of text to quote in your own work.
Text recognition is never 100% accurate. Many parts of the scanned page may not be reflected in the OCR text output, including: images, page layout, certain fonts or handwriting.
32
THE ADVANCE OF PHOTOGRAPHY
K K, so that the part of the negative which is to be printed lies perpendicularly under it. This part is then acted upon by the broad perpendicular bundles of light S S, and intensely coloured, while the marginal parts lying under the mask are affected only by the narrow slanting pencils, S' S', and therefore are coloured less intensely in proportion to their distance from the hole of the mask. Thus a gently vanishing margin is produced, looking very artistic, and yet only the result of a very simple trick of art.
The First Dry Plate. — As the chemical action of light upon the salts of silver became better known, it was found possible to prepare a sensitive plate and to delay using it
f J S' jf'
K
Lj
a L i
Fig. 9.
until after it was dry, and, in fact, had been kept some time. It was discovered that if bodies were present which could unite with the iodine of the silver iodide, then that substance was decomposed very much quicker by light than when the pure salt alone was present. Among these bodies were found to be such substances as extract of coffee or tea, morphine, and tannin, and it was by the aid of these that the early photographic dry plates were prepared. The method adopted with these plates was to wash away the nitrate of silver adhering to the moist plate, and then to coat the plate with a solution of either tannin or morphine. Such coatings can dr}7 up without injury to the film of iodide of silver, and so a durable dry plate was obtained. The sensitiveness of these plates was considerably less than that of the moist plates. The