A Manual Of Photography

94 HISTORY or PHOTOGRAPHY. after a few minutes, it is seen unfolding itself, and gradually becoming far more distinct than it was before ; delicate lines, before invisible, or barely seen, are now distinctly marked, and a rare and singular perfection of detail given to the drawing. It may appear, at first sight, that the bichloride of mercury dissolves off the metal, and again deposits it in the form of chloride (calomel). But this does not account for the fact, that if the paper has been prepared with the nitrate of silver, the mercury disappears, and the drawing vanishes, the deposit taking place only on those parts upon which light has acted but feebly ; as, for instance, on the venations of leaves, leaving those portions of surface which were exposed to lull luminous influence without a particle of quicksilver. When the paper has been either a chloride or iodide, the effect is as above, the thickness of the deposit is as the intensity of the light bas been ; consequently, the semi-tints are beautifully preserved. If the drawing remains too long in the solution, the precipitate adheres to the dark parts and destroys the effect. The singula- rity of this operation will be more striking if the picture has been soaked some time in the solution of the hyposulphite of soda, and then dipped into the bichloride of mercury. As the drawing disappears, a series of circles, formed of a white powder, appear to arise from the paper, generally commencing at the centre, and slowly extending over the whole surface . the powder is afterwards deposited, and the sheet is buried in the precipi- tate; but on taking the paper from the liquid, and passing a stream of water over it, the precipitate is entirely removed from all the parts except the lights of the picture. I have also found the invisible photographic image become evident, without the aid of mercurial vapour, by simply soaking for some time in a solution of corrosive sublimate. When these papers are prepared with due care, they are ex- tremely sensitive, and if used for copying engravings during bright sunshine, the effect is instantaneous. The great difficulty is to present the paper to the sun, and withdraw it with suffi- cient celerity In the weak light of the camera a few minutes du±sunshine is quite sufficient for the production of the best effects. One great advantage of these pictures over those pro- cured on the plated copper is, that the mercury does not he loosely as on the tablets, but is firmly fixed being absorbed by the paper ; therefore these pictures may be kept without injury in a portfolio. , .„, If, instead of immersing the paper in a vessel full of sulphu- retted hydrogen gas, a stream of the gas is made to play upon it, it assumes a most richly iridescent surface ; the various colours