ready to yield to the slightest extraneous force, such as thefeeble impact of the violet rays when much attenuated. Having,therefore, prepared a number of sheets of paper with chemicalproportions slightly different from one another, let a piece be cutfrom each, and, having been duly marked or numbered, let thembe placed, side by side, in a very weak diffused light for a quarterof an hour. Then, if any one of them, as frequently happens,exhibits a marked advanfage over its competitors, I select thepaper which bears the corresponding number to be placed in thecamera obscura.”
The increased sensitiveness given to paper by alternate ablu-tions of saline and argentine washes, the striking differences ofeffect produced by accidental variations of the proportions inwhich the chemical ingredients are applied, and the spontaneouschange which takes place, even in the dark, on the more sensitivevarieties of the paper, are all subjects of great interest, whichdemand 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-tainty, particularly those which appear to show the influence oftime, an element not sufficiently taken into account, in over-coming the weaker affinities. Few fields of research promise agreater measure of reward than these; already the art of makingsun pictures has led to many very important physical discoveries,but most of the phenomena are yet involved in obscurity.
SzcrioN II.—TuE CALOTYRE.
Although, in order of date, the investigations of Sir John Herschel and others have a priority over those particular expe-riments of Mr. Talbot’s which resulted in the discovery of hisvery beautiful process, the calotype, yet to avoid confusion it isthought advisable to group together the discoveries of each in-vestigator, where this is practicable, in our historical division.
The earliest productions of Mr. Talbot were simply such pre-parations as those already described, in which a chloride of silverwas formed on the surface of the paper, with some nitrate ofsilver in excess. These need not be any further described thanthey have already been.
Tarly in 1840, drawings on paper were handed about in thescientific circles of London and of Paris , which were a great ad-vance upon anything which had been previously done. Thesewere the results of a new process discovered by Mr. Talbot, and