2G6 
POPULAR SCIENCE REVIEW. 
picture can at once be made to furnish the cliche , as it is termed by the 
inventoi-, merely on printing by superposition. In order to prepare the 
glass or poi’celain surface to receive its impi-ession, it is coated with a 
layer of honey axid gelatine or albumen, containing a certain proportion of 
a soluble chromate, and when dried is placed in close contact with the 
cliche of the subject desired. By the unequal action of the suix’s rays 
consequent on exposing it to the light, the sensitive surface is differently 
affected, and the character of the preparation becomes so altered in those 
parts where the light has acted fully, that being now rendered quite dry 
and horny, it is iixcapable of retaining any of the powdered enamel colour 
which in the next operation is freely dusted over the surface. But in other 
portions of the design, where the chemical substances have been altogether 
protected, or only partially affected during the exposure to light, the 
glutinous character of the surface remains unaltered or becomes modified 
in proportion to the activity of the rays operating upon the sensitive 
preparation. It is then apparent that the plate, after exposure, will take 
the enamel coloui-s more or less freely when this lxiaterial in the state of 
fine powder is spread over the surface, and that by a judicious exercise of 
artistic skill it would be possible to modify not only the depth of tint 
produced by oixe and the same pigment, but also to apply different enamel 
colours over the several portions of the picture, and so obtain all the 
requirements of a coloured photograph on the same plate. It remains, 
finally, to burn in and develope the colours applied to the glass surface by 
a carefully conducted furnace treatment, which will also have the effect 
of destroying the organic substances originally employed in the prepara- 
tion. 
The discussion relative to the composition of the photographic image has 
been again revived by Mr. Malone, at the London Photographic Society. 
The lectui-er referred to the experiments of Scheele, Mulder, Hunt, and 
Spiller, and to the corroboration afforded by his own experimental results 
in support of the view which attributes to sunlight the power of decom- 
posing the chloride and similar compounds of silver into their component 
elements, the reduced metallic silver being liberated in so finely divided a 
condition that the dark shades of colour observed in the photograph were 
accounted for by the feeble light-reflecting capacity of these minute 
particles, and that the different tints obtained were solely dependeixt upon 
slight variations in physical structure, and had no reference to chemical 
composition. 
The mounting of photographs* receixtly formed the subject of an intei’esting 
papei", read by Mr. G. Wharton Simpson, at a meeting of the South 
London Photographic Society. After referring to the artistic and general 
considerations which should be kept in view when proceeding to lxxount a 
photographic print, the author treats of the relative efficiency and 
permanence of the several adhesive materials ordinarily employed in the 
operation of mounting. Enumei-ating the following substances — gum 
arabic, starch, dextrine, albumen, glue, gelatine, and isinglass, and for 
special purposes india-rubber paste prepared either with benzole or 
* Or photogra/ms, as they have recently been called. 
