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.
THE CHEMICAL ACTION OF LIGHT 63
Pure iodide of silver was formerly used in photography, but a mixture of iodide and bromide of silver is now preferred. This change was made because it was soon perceived that iodide of silver is very sensitive to strong light, but by no means so to weak light. For example, in taking a portrait, iodide of silver gives the light parts, such as the shirt and the face, in a few seconds with great clearness ; whereas the darker parts, such as the shadows, the dark coat, etc., are very feebly given. But if some bromide of silver is mixed with the iodide, the coating of combined iodide and bromide of silver gives a weaker but still intense picture of the clear parts, while it gives a much better impression of the dark parts than iodide of silver alone.
The mixture of iodide and bromide of silver was effected in practice in the case of the wet plates by adding to the collodion a salt containing iodine and a salt containing bromine ; for example, iodide of potassium and bromide of cadmium. Both are decomposed in the silver bath. Iodide of potassium and nitrate of silver produce iodide of silver and nitrate of potash, and in the same way bromide of cadmium and nitrate of silver produce bromide of silver and nitrate of cadmium.
Development of the Wet Plate. — In the case of the old wet plates a considerable quantity of the solution of silver remains also adhering mechanically to the collodion coating. This adhering solution of silver is by no means a matter of secondary importance ; on the contrary, while developing, it affords the necessary material from which the fine silver powder is precipitated.
If the developer (for example, a solution of green vitriol) is mixed with a solution of silver, the silver is precipitated in the form of a fine powder. For green vitriol — protosulphate of iron — readily absorbs oxygen, and is changed thereby into persulphate of iron. Accordingly, if a body containing oxygen (for example, nitrate of silver) is mixed