intensity and duration of their exposure to light, resulting in a visible image. The later and more convenient dry process was similar but used a gelatin emulsion which could be applied to the plate long before use and exposed in the camera dry. In both processes, a very underexposed negative image was produced in the emulsion. Its densest areas, corresponding to the lightest parts of the subject, appeared gray by reflected light. The areas with the least amount of silver, corresponding to the darkest areas of the subject, were essentially transparent and appeared black when seen against the dark background provided by the lacquer. The image as a whole therefore appeared to be a dull-toned positive. This ability to employ underexposed images allowed shorter exposure times to be used, a great advantage in portraiture. To obtain as light-toned an image as possible, potassium cyanide was normally employed as the photographic fixer. It was perhaps the most acutely hazardous of all the several highly toxic chemicals originally used in this and many other early photographic processes. One unusual piece of tintype equipment was a twelve-lensed camera that could make a dozen "gem" portraits with one exposure, developed in 1858. Portrait sizes ranged from gem-size to . From about 1865 to 1910, the most popular size, called "Bon-ton", ranged from to . Each tintype is usually a camera original, so the image is usually a mirror image, reversed left to right from reality. Sometimes the camera was fitted with a mirror or right-angle prism so that the result would be right-reading. History The process was first described by Adolphe-Alexandre Martin in France in 1853. In 1856 it was patented by Hamilton Smith in the United States and by William Kloen in the United Kingdom. It was first called melainotype, then ferrotype by V.M. Griswold of Ohio, a rival manufacturer of the iron plates, then finally tintype. Ambrotype as a precursor The ambrotype was the first use of the wet-plate collodion process as a positive image. Such collodion glass positives had been invented by Frederick Scott Archer in 1851. Although it is a widely held belief James Ambrose Cutting might have named the process after himself, in actuality, "ambrotype" was first coined by Marcus Aurelius Root, a well known daguerreotypist, in his gallery as documented in the 1864 book The Camera and the Pencil. The tintype was essentially a variant of the ambrotype,