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 468 PHOTOGRAPHY and as a literary critic. Though he did not consummate the separation between the Greek and Latin churches, he created a division which was never healed, and after him, with a few transient exceptions, no confession of suprem- acy could be wrung by the pope from the Greek patriarchs. He drew up charges against the Latin church, that they shortened the sea- son of Lent, refused to allow married men to enter the priesthood, and denied to priests the right to administer the chrism, and above all that they taught the double procession of the Holy Spirit. Of his numerous works, the most important is the Bibliotheca, which contains fragments of nearly 300 Greek prose writers, most of whose works are lost, with critical re- marks thereon. Editions of this work have been published in Augsburg (1601), in Geneva, with a Latin translation (1612), and in Ber- lin by Bekker (1824r-'5). He also left a " Lexi- con" (Leipsic, 1808; London, 1822); the "No- mocanon," a collection of canonical decrees, epistles, and statutes concerning the church (Paris, 1615) ; a collection of 248 letters (Lon- don, 1651); theological tracts, contained in Oombefis's supplement to the Bibliotheca Pa- trum ; and a treatise on " Consolation," edited by Rittershusius (Nuremberg, 1601). Some additional fragments of his writings are con- tained in the collection published by Cardinal Mai in 1825-"T from the MSS. in the Vatican. An excellent monograph on Photius, by the abbe Jager, was published at Paris in 1845. See also Hergenrother, Photius von Konstanti- nopel (3 vols., Ratisbon, 1867-'9). PHOTOGRAPHY (Gr. ^6f, light, and -ypdfeiv, to write), the art of depicting objects by the agency of light. The earliest observations on the chemical changes produced by the agency of light were doubtless those of the fading and bleaching of vegetable colors. Of the delicate tints that may be obtained from the parts of plants, some are so sensitive to light that an exposure of only a few moments is sufficient to injure them ; others resist for a longer time the conjoint action of sunlight, air, and water. It was noticed by the alche- mists, probably about the 12th century, that the chloride of silver blackens by exposure to the sun, though when first prepared it is as white as snow. This darkening is in a general man- ner proportional to the brightness of the light. It does not occur instantaneously, but in a regulated way, a given quantity of light be- ing apparently necessary for the production of a definite effect. As experimental chemistry was cultivated, the list of substances thus in- fluenced became greatly extended, and when Berzelius published his work on chemistry sev- eral scores of bodies were known to be change- able by luminous agency. Some of these were elementary, and some were compounds, de- rived from both the inorganic and organic groups. Perhaps the first germ of photogra- phy as an art is presented in an experiment of Priestley's, who caused some chloride of silver to be deposited on the side of a glass bottle, and then putting round the bottle a piece of dark paper out of which letters had been cut with a penknife, the arrangement was exposed to the sun. All those portions of chloride upon which the light had fallen, through the spaces where the paper had been removed, turned black, but those protected by the dark paper retained their whiteness unimpaired. Scheele also made some very instructive experiments for the purpose of determining whether it was some specially colored ray of light, or light in the aggregate, that produced the result. He caused a beam to enter a darkened chamber through a hole in the window shutter, as in Newton's experiment for the decomposition of light, and, intercepting the beam by means of a glass prism, dispersed it into its constituent rays. The colored spectrum thus produced was received on a sheet of paper painted over with chloride of silver. The blackening began in the indigo or violet region, and extended in the more refrangible direction far beyond the limits of visibility. In the other direction it stopped short in the blue space, so that the green, the yellow, the orange, and the red ex- hibited no kind of action. From this it ap- pears that a sunbeam does not darken the chloride of silver in virtue of its light, but that the decomposition is brought about by some other principle contained in the beam, con- joined with the light, and found to the greatest degree in the more refrangible end of the spec- trum. To the rays thus recognized as occa- sioning the changes the designation of chemical rays was given, and they were likewise called deoxidizing rays. In observations upon the sun with reflecting telescopes, Sir William Herschel had been obliged to use colored glass screens, for the purpose of diminishing the exces- sive brilliancy of the light, and had accidental- ly noticed that the heat transmitted through these colored glasses was very far from be- ing proportional to the light. A glass colored deeply enough to absorb a large portion of the light rays might nevertheless transmit an unexpected proportion of the heat rays. He therefore prepared a solar spectrum after the manner of Newton, and set in each of its col- ored spaces the bulb of a delicate thermome- ter. Starting from the violet and descending through the indigo, the blue, green, yellow, orange, and red, the thermometer stood higher and higher ; outside of the red and beyond the visible limits of the light, it stood highest of all. No other interpretation could apparently be given to such an experiment than that the heat and the light are altogether independent agents, and distributed very differently in the spectrum. This conclusion contained, how- ever, a most important error, which was per- petuated until a later period in these discover- ies ; it overlooked the physiological peculiari- ties of an organ of vision like the human eye. It also overlooked the physical fact that the colored spaces from the violet to the red un-