INDEX OF NAMES. [The names of persons by whom inventions have been communicated from INTRODUCTION. In order to render the Abridgments of Specifications in which Photography is referred to as complete as possible, the following brief summary of the progress of knowledge in reference to this subject is prefixed. By the chronological arrangement of the matter, the treating of each discovery or invention in a separate paragraph, and the citation of the works consulted, it has been sought to make the heads of the subject easy of reference, and to show readily where more detailed information can be found. Although the action of light on coloured bodies must have been of every-day occurrence, the philosophers of antiquity did not record the fact, and the chemical action of light upon matter would appear to have entirely escaped their observation. The first definite knowledge of this action appears to have been some observations of the alchemists, that "horn silver" (chloride of silver) was blackened by exposure to light. Although the middle ages thus furnished the discovery of one of the facts upon which the scientific principles of photography depend; the others-viz., the proper application of designs or images to sensitive surfaces and the permanent fixing of the pictures so obtained-are belonging to the present era. B.C. 300. EUCLID, about 300 B.C., in his "Treatise on Optics" (26th, A.D. 27th, and 28th theorems) proves that he knew that the pictures of bodies seen by both eyes are formed by the union of two dissimilar pictures formed by each eye. (See Brewster on the Stereoscope, p. 6; also Euclid's Optics, Edit. of Pena, pp. 17, 18, Paris, 1577; or Opera, by Gregory, pp. 619, 620, Oxon. 1703.) 170. GALEN, about A.D. 170, in the twelfth chapter of the tenth book of his work, "De Usu Partium Corporis Humani," described the phenomena attendant upon looking at |