The Art of Photography, which has now attained such perfection, and has become so popular amongst all classes, is one of comparatively recent introduction. The word Photography means literally "writing by means of Light;" and it includes all processes by which any kind of picture can be obtained by the chemical agency of Light, without reference to the nature of the sensitive surface upon which it acts. The philosophers of antiquity, although chemical changes due to the influence of Light were continually passing before their eyes, do not appear to have directed their attention to them. Some of the Alchemists indeed noticed the fact that a substance which they termed "Horn Silver," which was probably a Chloride of Silver which had undergone fusion, became blackened by exposure to Light; but their ideas on such subjects being of the most erroneous nature, nothing resulted from the discovery. The first philosophical examination of the decomposing action of Light upon compounds containing Silver was made by the illustrious Scheele, no longer than three-quarters of a century ago, viz. in 1777. It was also remarked by him that some of the coloured rays of Light were peculiarly active in promoting the change.