A Manual Of Photography

Talbot’s photogenic drawings. CALOTYPE. 21 icady to yield to the slightest extraneous force, such as the ^edle impact of the violet rays when much attenuated. Having, let of ore, prepared a numher of sheets of paper with chemical proportions slightly different from one another, let a piece he cut rem each, and, haying been duly marked or numbered, let them be placed, side by side, in a very weak diffused light for a quarter O on^ Then, if any one of them, as frequently happens, exhibits a marked advantage over its competitors, I select the paper winch bears the corresponding number to be placed in the The increased sensitiveness given to paper by alternate ablu- ions ol sahne and argentine washes, the striking differences of $uced, by accidental variations of the proportions in w ic i the chemical ingredients are applied, and the spontaneous ange which takes place, even in the dark, on the more sensitive varieties of the paper, are all subjects of great interest, which demand further investigation than they have ever yet received, and which, if followed out, promise some most important expla- nations of chemical phenomena at present involved in uncer- ainty, particularly those which appear to show the influence of time, an element not sufficiently taken into account, in over- coming the weaker affinities. Tew fields of research promise a greater measure of reward than these ; already the art of making sun pictures has led to many very important physical discoveries, but most of the phenomena are yet involved in obscurity. Section II.— The Calotype. U^^ongh, in order of date, the investigations of Sir John lierschef and others have a priority over those particular expe- iiinen s of Mr. Ialbot’s which resulted in the discovery of his very beautiful process, the calotype, yet to avoid confusion it is thought advisable to group together the discoveries of each in- vestigator, where this is practicable, in our historical division. Ihe earliest productions of Mr. Talbot were simply such pre- parations as those already described, in which a chloride of silver was formed on the surface of the paper, with some nitrate of silver in excess. These need not be any further described than they have already been. Tarty in 1840, drawings on paper were handed about in the scientific circles of London and of Paris, which were a »Teat ad- vance upon anything which had been previously done? These were the results of a new process discovered by Mr. Talbot, and