A Manual Of Photography

280 PRACTICE OF PHOTOGRAPHY. the change of a Talbotype negative into apparently a positive Daguerreotype, the positive still retaining its negative properties when viewed by transmitted light. To fix the picture, a solution of one part of hyposulphite 01 soda in sixteen parts of water is poured upon the plate, and e for several minutes, until the iodide of silver has been dissolved. Washing in water completes the process. “The phenomenon of the Daguerreotype, says Mr. Malone, 6‘ is in this case produced by very opposite agency, no mercury being present ; metallic silver here producing the lights, while in the Daguerreotype it produces the shadows of the picture. We at first hesitated about assigning a cause for the dull white granular deposit which forms the image, judging it to be due simply to molecular arrangement. Later experiments, however, have given us continuous films of bright metallic silver and we find the dull deposit becomes brilliant and metallic when bur- nished. It should be observed that the positive image we speak of is on glass, strictly analogous to the Daguerreotype. It is positive when viewed at any angle but that which enables it to reflect the light of the ray. This is one of its characteristics. It must not be confounded with the continuous film image which is seen properly only at one angle ; the angle at which the other ceases to exist. It is also curious to observe the details of the image, absent when the plate is viewed negatively by transmitted light, appear when viewed positively by reflected light. Section III.—Mr. Mayall’s Process. Mr. Mayall has recently published a form of process, employed by M. Martin, which differs in no essential particular from those already described; but as involving some niceties of manipu- lation, on which, the writer says, depends the perfection of his finished pictures, it is thought advisable to quote P « First. The albumen of a fresh egg must be beaten into a snow-like mass with a bunch of quills, dropping into it ten drops of a saturated solution of iodide of potassium ; allow it to stand six hours in a place free from dust, and moderately warm,—say " Second. A piece of hand-plate glass, eight inches by six, with the edges ground smooth, must be cleaned as follows: with a piece of cotton wool rub over both sides with concentrated nitric acid, then rinse well with water, and dry. Stick a water on that side which I will now call the back, to mark it; pounce upon