A Manual Of Photography

51 HISTORY Of PHOTOGRAPHY. notice. Nevertheless, if a slip of this paper be held for only four or five seconds in the sun (the effect of which is quite im- perceptible to the eye), and when withdrawn into the shade be washed over with the ferrosesquicyanate of potash, a conside- rable deposit of Prussian blue taltes place on the part sunned, and none whatever on the rest ; so that on washing the whole with water, a pretty strong blue impression is left, demonstrat- ing the reduction of iron in that portion of the paper to the state of protoxide. The effect in question is not, it should be observed, peculiar to the ammorio-citrate of iron. “The ammonio- and potassio-tartrate fully possess, and the perchloride exactly neutralized, partakes of the same property: but the experiment is far more neatly made, and succeeds better, with the other salts." In further development of these most interesting processes Sir John Herschel says:—“ The varieties of cyanotype pro- cesses seem to be innumerable, but that which I shall now describe deserves particular notice, not only for its pre-eminent beauty while in progress, but as illustrating the peculiar power of the ammoniacal and other persalts of iron above mentioned to receive a latent picture, susceptible of development by a great variety of stimuli. This process consists in simply passing over the ammonio-citrated paper on which such a latent picture has been impressed, very sparingly and evenly, a wash of the solution of the common yellow ferrocyanate (prussiate) of potash. The latent picture, if not so faint as to be quite in- visible (and for this purpose it should not be so), is negative. As soon as the liquid is applied, which cannot be in too thin a film, the negative picture vanishes, and by very slow degrees is replaced by a positive one of a violet-blue colour on a greenish- yellow ground, which at a certain moment possesses a high degree of sharpness, and singular beauty and delicacy of tint. If at this instant it be thrown into water, it passes immediately to Prussian blue, losing at the same time, however, much of its sharpness, and sometimes indeed becoming quite blotty and confused. But if this be delayed, the picture, after attaining a certain maximum of distinctness, grows rapidly confused, es- pecially if the quantity of liquid applied be more than the paper can easily and completely absorb, or if the brush in applying it be allowed to rest on, or to be passed twice over any part. The effect then becomes that of a coarse and ill-printed woodcut, all the strong shades being run together, and a total absence pre- vailing of half lights. “To prevent this confusion, gum-arabic may be added to the prussiated solution, by which it is hindered from spreading un-