Volume 2

Ure's dictionary of arts, manufactures, and mines : containing a clear exposition of their principles and practice / by Robert Hunt. ; assisted by F.W. Rudler.

Ure, Andrew, 1778-1857.

Date:: 1878-1979

Credit: Ure's dictionary of arts, manufactures, and mines : containing a clear exposition of their principles and practice / by Robert Hunt. ; assisted by F.W. Rudler. Source: Wellcome Collection.

19/1060 (page 3)

$favourable light, to produce the desired effect. Numerous modiBcations were speedily iutrodiiced, and many of them were patented. The proeressivo advance of this branch of the photographic art, though of great interest, cannot be dwelt on in this place. Those who are interested m the inquiry ^v•ilI find the information fully detailed in Hunt's 'Manual of Photography, las_. edition, and more recent works. It will be sufficient here to name the more important improve- monts which have become generally adopted. The first advance of real importance was made bv Mr. Towson, of Devonport, who has since that time distinguished himself by the introduction of his system of Groat Circle Sailing. Mr. Towson suggested the use of enlarged lenses ; and by acting with such Dr. Draper, of New York was the first to procure a portrait from the Hfe. Still this was a tedious process ; but in 1840 Mr Goddard proposed the use of bromide of iodine, by which mfinitoly increased sensibility was obtained. From that time the Daguerreotype was generally employed for portraiture, until the facilities of the Collodion_ process drove it from the held. The improved manipulation now resolves itself into :— _ Carefully polishing the silver plate after some of the methods previously described, and the application finally of the highest poUsh by the use of a buffer, the best form being that employed by M. Claudet. . , -, , • r. In a box on a roller, to which there is a ha^ndle, Jiff. 596, is placed a long piece of drab-coloured velvet, which can be drawn out and extended, by means of a second 596 roller, upon a perfectly flat table. The fii-st foot or two, for example, is drawn out; the plate which has already received its preliminary polishing is placed face down- wards, and being pressed close with the fingers, a rapid circular motion is given to it, and in a few minutes it receives its highest lustre. As the velvet becomes blackened by use, it is rolled off, the portion remaining in the box being always perfectly clean and ready for use. . The iodising process follows : and for this purpose a box similar to that representea will be found to be very convenient {Jiff. 597). This iodising apparatus consists of a square box, with a closely-fitting square cover g: 5j7 false sides are placed at an angle with this box; a cup D at the bottom contains the iodine, which is covered with a thin gauze screen j j. c is a cover which confines the iodine when it is not required for the plate; this dividing the box into two parts, H H and k k, the former being always fiiU of iodine vapour. When it is desired to iodise a plate, the cover c is removed, the silver plate is placed at e, and the cover G closed. The plate is thus placed in the iodine box until it acquires a fine straw-yellow eoloui. In another box is placed either bromine or some of the many accelerating fluids. If bromine, or any bromide is employed, the plate should remain until it is of a rose colour. As a general rule, if the yellow colour produced by iodine be pale, the red should bo pale also; if deep, the red must incline to violet. The proper time for ex- posing a plate to any of those chemical substances which are destined to produce the sensitive film must vary with the temperature, and it can only be determined hy experience. The sensitive plate is now removed to the camera obscura, for a descrip- tion of whicli see Photography. It is scarcely necessary to say, that the plate must be preserved in perfect darkness until exposed to the image in the camera. A few seconds wlien the plate is properly prepared will be found amply sufficient to produce the best effect. The impression must be developed in the mercury box {Jig. 598) in the manoor b3$