" On examining the transferred image by reflected light, it will appear as a faint and somewhat shadowy transcript of the original drawing, in which a careful inspection in a favourable light will detect many details re-produced with great sharpness and delicacy. By transmitted light, however, the semi-transparent nature of metallic films pf extreme tenuity will be found painfully evident. It is indeed a faint negative, but it differs from those obtained by ordinary processes in two most important particulars. In the first place its lights are perfectly and absolutely pure, and in the second its half tints, however faint, are all represented by a metallic equivalent, really and substantially existing on the surface of the gelatine, and which, therefore, may become the basis of a chemical action, although too minute to be detected by the most careful inspection. The colour of the metallic film varies greatly, generally approaching to a reddish brown where it is most dense. This clearly points out a fact for which we should hardly have been prepared, viz., that the high lights in Daguerre's image are in reality formed of two distinct layers, the upper stratum being blanched by the action of the mercury, and probably amalgamated with it, whilst the lower retains the reddish hue which reduced silver sometimes assumes. The rosy tint which is observable in the high lights of the finest proofs, when seen obliquely, is thus explained. Considered as a basis of chemical action, the transferred image is a sheet of gelatine, on which particles of pure metallic silver, or of silver amalgam, are more or less densely strewn. To increase the opacity of these particles, so as to render
them less permeable to the rays of transmitted light, is the problem still to be solved, and for the solution of which three methods are open :—First; to transform the metallic particles into some binary compound, such as an oxide, a sulphuret, an iodide, or a chloride ; secondly, to substitute for them thin chemical equivalents of platinum or gold ; and thirdly, to render them the centre of a catalytic action, which shall group around them fresh molecules of reduced silver. The very few experiments which I have been able to make in these different directions have impressed me with the belief that no very serious obstacle is to be apprehended. Thus I have found that the action of iodine transforms the metallic film into a saffron-coloured compound which is not altered by exposure to light. Bi-chloride of mercury changes it into a greyish powder, which is again darkened by a weak solution of ammonia, and the terchloride of gold increases considerably the intensity of the image, but forms unfortlinately, with the gelatine, a compound of a truly Tyrean purple tint.
" All these, however, are topics on which I need not dwell, as they will naturally suggest themselves to the minds of those who may deem the subject worthy of investigation."