A Manual Of Photography

SIE J. HEBSCHEL’s CYA^OTYtE. 53 "If paper be washed with a solution of ammonio-citrate ot iron, and dried, and then a wash passed over it of the yellow ferrocyanuret of potassium, there is no immediate formation ot true Prussian blue, but the paper rapidly acquires a violet- purple colour, which deepens after a few minutes, as it dries, to almost absolute blackness. In this state it is a positive photo- graphic paper of high sensibility, and gives pictures of great depth and sharpness ; but with this peculiarity, that they darken again spontaneously on exposure to the air in darkness, and are soon obliterated. The paper, however, remains susceptible to light, and capable of receiving other pictures, which in their turn fade, without any possibility (so far as I can see) of arresting them; which is to be regretted, as they are very beautiful, and the paper of such easy preparation. If washed with ammonia or its own carbonate, they are for a few moments entirely oblite- rated, but presently reappear, with reversed lights and shades. In this state they are fixed, and the ammonia, with all that it will dissolve, being removed by washing in water, their colour becomes a pure Prussian blue, which deepens much by keeping If the solution be mixed, there results a very dark violet-coloured, ink, which may be kept uninjured in an opaque bottle, and will readily furnish, by a single wash, at a moment’s notice, the positive paper in question, which is most sensitive when wet. " It seems at first sight natural to refer these curious and complex changes to the instability of the cyanic compounds; and that tins opinion is to a certain extent correct, is proved by the photographic impressions obtained on papers to which no iron has been added beyond what exists in the ferrocyanic salts themselves. Nevertheless, the following experiments abun- dantly prove that in several of the changes above described, the Immediate action of the solar ray s is not exerted on these salts, but on the iron contained in the ferruginous solution added to them, which it deoxidizes or otherwise alters, thereby presenting it to the ferrocyanic salts in such a form as to precipitate the acids in combination with the peroxide, or protoxide of iron, as the case may be. To make this evident, all that is necessary is simply to leave out the ferrocyanate in the preparation of the paper, which thus becomes reduced to a simple washing over with the ammonio-citric solution. Paper so washed is of a bright yellow colour, and is apparently little, but in reality highly sensitive to photographic action. Exposed to strong sunshine, for some time indeed, its bright yellow tint is dulled into an ochrey hue, or even to grey, but the change altogether amounts to a moderate per-centage of the total light reflected, and in short exposures is such as would easily escape