A Manual Of Photography

206 PRACTICE OF PHOTOGRAPHY. attempt to cover a spot which has not been wetted, by the application of fresh solution to the place, as it will, in darkening, a ________________become a well-defined space of a different A A A A shade from the rest of the sheet. The only plan is, when a space has escaped our atten- tion in the first washing, to go over the whole sheet with a more dilute solution. It is, indeed, always the safest course to give the sheet two washings. The nitrated paper not being very sen- sitive to luminous agency, it is desirable to U U U U increase its power. This may be done to * some extent by simple methods. 591 By soaking the paper in a solution of isinglass or parchment size, or by rubbing it over with the white of egg, and drying it prior to the application of the sensitive wash, it will be found to blacken much more readily, and assume different tones of colour, which may be varied at the taste of the operator. By dissolving the nitrate of silver in common rectified spirits of wine, instead of water, we produce a tolerably sensitive nitrated paper, which darkens to a very beautiful chocolate brown ; but this wash must not be used on any sheets prepared with isinglass, parchment, or albumen, as these substances are coagulated by alcohol. The nitrate of silver is not sufficiently sensible to change readily in diffused light ; consequently it is unfit for use in the camera obscura, and it is only in strong sunshine that a copy of an engraving can be taken with it. Ammonio-Mtrate of Silver.—This is an exceedingly useful preparation for many purposes. It is prepared by adding ammonia to a solution of nitrate of silver: a deep olive preci- pitate of oxide of silver takes place ; more ammonia should then be added, drop by drop, until this precipitate is redissolved, great care being taken care that more ammonia is not added than is necessary to do this. This solution is more sensitive than the nitrate, and may be used with advantage for copying by superposition ; but it is not fitted for the camera obscura. Section II.—Chloride of Silver. This is one of the most important salts employed in photo- graphy : it therefore demands especial attention.