A Manual Of Photography

ON FIXING THE PHOTOGRAPHIC PICTURES. 21/ useful : they were first indicated by Sir Jolin Herschel, from whose memoir on the “ Chemical Agency of the Hays of the Solar Spectrum” I quote : — , “ By far the most remarkable fixing process with which 1 am acquainted, however, consists in washing over the picture witn a weak solution of corrosive sublimate, and then laying it for a few moments in water. This at once and completely oblite- rates the picture, reducing it to the state of perfectly white paper, on which the nicest examination (if the process be perfectly executed) can detect no trace, and in which it may be used for any other purpose, as drawing, writing, Ac., being completely insensible to light. Nevertheless, the picture, though invisible, is only dormant, and may be instantly revived in all its force by merely brushing it over with a solution of a neutral hyposulphite, after which, however, it remains as insensible as before to the action of light. And thus it may be successively obliterated and revived as often as we please. It hardly requires mention that the property in question furnishes a means of painting in mezzotinto (/. e. of commencing on black paper and working in the lights), as also a mode of secret writing, and a • variety of similar applications.. “There is a remarie which ought not to be omitted in regard to this part of our subject-viz., that it makes a great différence, in respect of the injury done to a photographic picture by the fixing process, whether that picture have been impressed by the long-continued action of a feeble light, or by the quick and vivid one of a bright sun. Even supposing the pictures originally of equal intensity, the half-tints are much less power- fully corroded or washed out in fixing in the latter case than in the former.”