24

NEGATIVE PHOTOGRAPHS.

manipulation produce extreme sensitiveness, but mucb exact attentionis required to prevent any excess of muriatic acid, which, in the state ofvapour, is rapidly absorbed by the paper. The whole of tlio nitrato ofsilver employed in the first instance, must be converted into a muriate,and there tli? process should stop.

The Doctor’s method of fixing, which I givo here to avoid confusion,is also extremely difficult. The drawing is to be steeped for five or tenminutes in alcohol, and, after removing all superfluous moisture bymeans of blotting paper, and drying it slightly before the fire, the paperthus prepared is drawn through diluted muriatic acid, mixed with afew drops of an acid nitrate of quicksilver, prepared by dissolvingquicksilver in pure nitric acid, and again dissolving the crystallised saltto saturation in water acidulated witli nitric acid. The addition of thenitrate of mercury requires great caution, and its proper action must betried first on slips of paper, upon which have been produced differenttints and shadows by exposure to light; because if added in too great aquantity, tho lightest shades entirely disappear. The paper havingbeen drawn through tho above mentioned solution, is well washed inwater, and then dried in a degree of heat approaching to about 158°Fahr., or, in fact, till tho white places assume a very slight tinge ofyellow. The appearance of this tint indicates that the drawing is fixedpermanently.

The author speaks of reversing these drawings by transfers, which Ido not understand, and have never succeeded in doing with any thinglike a good effect. The yellow tinge induced on the paper to give thedrawing permanence, most effectually provents tho permeation of thoserays of light which blacken the sensitive chloride of silver.

D.—On the Methods of Using the Photographic Papers preparedwith the Salts of Silver . Negative kind.

a. On taking copies of Botanical Specimens, Engravings, §c.

For the multiplication of photographic drawings, it is necessary tobe provided with a frame and glass, the most convenient size for jwhich is about that of a single leaf of quarto post writing paper. Theglass must be of such thickness as to resist some considerable pressure,and it should be selected as colourless as possible, great care beingtaken to avoid such as have a tint of yellow or red, these colours pre-venting tho permeation of the most efficient rays. Figures 5 and Grepresent the frame, tho one showing it in front as in taking a copy ofleaves; and the other the back with its piece of stout tinned iron, whichpresses on a cushion, securing the close contact of the paper with everypart of the object to be copied, and its brass bar, which, when pressedinto angular apertures in the sides of the frame, gives the required pres-sure to the paper.