A Manual Of Photography

SIR J. HERSCHEL’S AMPHITYPE. 63 perceptible picture to one of the highest conceivable fulness and richness both of tint and detail, the colour in this case being a superb velvety brown. This extreme richness of effect is not produced except lead be present either in the ingredients used, or in the paper itself. It is not, as I originally supposed, due to the presence of free tartaric acid. 1 he pictures in this state are not permanent. They fade in the dark, though with very different degrees of rapidity, some (especially if free tar- taric or citric acid be present) in a few days; while others remain for weeks unimpaired, and require whole years for their total obliteration. But thougli entirely faded out in appear- ance, the picture is only rendered dormant, and may be re- stored, changing its character from negative to positive, and its colour from brown to black (in the shadows) by the following process :—A bath being prepared by pouring a small quantity of solution of per-nitrate of mercury into a large quantity of Water, and letting the sub-nitrated precipitate subside, the picture must be immersed in it (carefully and repeatedly clearing off the air-bubbles), and allowed to remain till the picture (if any where visible) is entirely destroyed, or if faded, till it is judged sufficient from previous experience ; a term which is often marked by the appearance of a feeble positive picture of a bright yellow hue on the pale yellow ground of the paper. A long time (several weeks) is often required for this, but heat accelerates the action, and it is often complete in a few hours. In this state the picture is to be very thoroughly rinsed and soaked in pure warm water, and then dried. It is then to be Well ironed with a smooth iron, heated so as barely not to injure the paper, placing it, for better security against scorching, between smooth clean papers. If, then, the process have been successful, a perfectly black positive picture is at once deve- loped. At first it most commonly flappens that the whole picture is sooty or dingy to such a degree that it is condemned as spoiled, but on keeping it between the leaves of a book, especially in a moist atmosphere, by extremely slow degrees this dinginess disappears, and the picture disengages itself with continually increasing sharpness and clearness, and acquires the exact effect of a copper-plate engraving on a paper more or less tinted with pale yellow. “ I ought to observe, that the best and most uniform specimens which I have procured have been on paper previously washed with certain preparations of uric acid, which is a very re- markable and powerful photographic element. Ilie intensity of tire original negative picture is no criterion of what may lie expected in the positive. It is from the production, by one and