A Manual Of Photography

188 PRACTICE OF PHOTOGRAPHY. the light of a lamp or candle at night. By extending the paper between the light and the eye, and slowly moving it up and down, and from left to right, the variations in its texture will be seen by the different quantities of light which permeate it ; and it is always the safest course to reject every sheet in which any inequalities are detected. By day it is more difficult to do this than at night, owing to the interference of the refleced with the transmitted light. It will, however, often happen that paper which has been carefully selected by the above means will imbibe fluids very unequally. In all cases where the paper is to be soaked in saline solutions, we have another method of discovering those sources of annoyance. Having the solution in a broad shallow vessel, extend the paper, and gradually draw it over the surface of the fluid, taking care that it is wetted on one side only. A few trials will render this perfectly easy. As the fluid is absorbed, any irregularities are detected by the difference of appearance exhibited on the upper part, which will, over well- deflned spaces, remain of a dull-white, whilst other portions will be shining with a reflective film of moisture. Where the im- portance of the use to which the paper is to be applied,—as, for instance, copying an elaborate piece of architecture with the camera, or for receiving the portrait of an individual, will repay a little extra attention, it is recommended that the paper be tried by this test with pure water, and dried, before it is submitted to the salting operation. It will be sometimes found that the paper contains minute fibres of thread, arising from the mass of which it is formed not having been reduced to a perfect pulp. Such paper should be rejected, and so also should those kinds which are found to have many brown or black specks, as they materially interfere with some of the processes. Some specimens of paper have an artificial substance given to them by sulphate of lime (plaster of Paris), but, as these are generally the cheaper kinds of demy, they are to be avoided by purchasing the better sorts. The plaster can be detected by fusing a sheet of the paper and examining the quantity of ash. Pure paper leaves less than £ per cent, of ash. If plaster is present the ash will be much more considerable : the increase of weight is, however, sometimes due to kaolin. No really sensitive paper can be pre- pared when sulphate of lime is present ; and it has the singular property of reversing the action of the hydriodic salts on the darkened chloride of silver, producing a negative in the place of a positive photograph. It is the custom for paper-makers to fix their names and the date on one leaf of the sheet of writing paper. It is wise to reject this leaf, or to select paper which is not so marked, as, in many of the photographic processes which will be described, these marks are brought out in most annoying