Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/857

P H O P H O 821 himself in the most favourable light, they nevertheless seem to afford sufficient testimony of a magnanimous spirit and a feeling heart. The most important of the works of Photius is his renowned Myriobiblion, a collection of extracts from and abridgments of 280 volumes of classical authors, the originals of which are now to a great extent lost. Dictated in haste immediately before his departure on his Eastern embassy, it is open to the charges of imper fect recollection and hasty criticism, but these are as nothing in comparison with its merits. It is especially rich in extracts from historical writers. To Photius we are indebted for almost all we possess of Ctesias, Mernnon, Conon, the lost books of Diodorus Siculus, and the lost writings of Arrian. Theology and ecclesi astical history are also very fully represented. The best edition is Bekker s (Berlin, 1824-25), which, however, has neither notes nor a Latin version. The next of his works in importance is the Amphilochia, a collection of 333 questions and answers on difficult points in Scripture, addressed to Amphilochius, archbishop of Cyzicus. This valuable work has exposed Photius to charges of plagiarism, which, as he does not claim entire originality, are wholly undeserved. The only complete edition is that published by Sophocles (Ecoiiomus at Athens in 1858. Photius is further author of a Lexicon (London, 1822), of a Nomocanon or harmony of the ecclesiastical canons with the imperial edicts relating to the discipline of the church, a work of great authority, but based on the labours of his predecessors, and of numerous theological writ ings. The more important of these are his treatise Against the Paulicians, in four books, and his controversy with the Latins on the procession of the Holy Spirit. His Epistles are valuable from their contents, but the style is often affected or unsuitable to the subject. The most complete edition is Valetta s (London, 1864). Many of Photius s works yet remain in manuscript. The only complete edition is Bishop Malou s in Migne s Patrologia Grfeca, and this is very imperfect and unsatisfactory. After the allusions in his own writings the chief contemporary authority for the life of Photins is his bitter enemy Nicetas the Paphlagonian, the biographer of his rival Ignatius. In modern times his life has been written with great prejudice and animosity by Baronius, and by Weguelin in the Memoirs of the Berlin Academy, and more fairly by Hankins (De Byzantinarum Rerum Scriptor- ibus, pt. 1). But all previous writers are superseded by the classical work of Cardinal Hergenrother, Photius, Patriarch von Constant inopel (3 vols., Ratisbon, 1867-69). As a dignitary of the Roman Catholic Church Cardinal Hergenrother is inevitably biassed against Photiu.s as an ecclesiastic, but his natural candour and sympathy with intellectual eminence have made him just to the man, while his investigation of all purely historical and literary questions is industrious and exhaustive in the highest degree. (R. G.) PHOTOGEAPHY IT would be somewhat difficult to fix a date when what we now know as "photographic action" was first re corded. No doubt the tanning of the skin by the sun s rays was what was first noticed, and this is as truly the effect of solar radiation as is the darkening of the sensitive paper which is now in use in photographic printing opera tions. We may take it that Scheele, the Swedish chemist, was the first to enter upon a scientific investigation of the darkening action of sunlight on silver chloride. He found by experiment that when silver chloride was exposed to the action of light beneath water there was dissolved in the fluid a substance which, on the addition of caustic (silver nitrate), caused the precipitation of new silver chloride, and that on applying liquor ammonia to the blackened chloride an insoluble residue of metallic silver was left behind. He also noticed that of the rays of the spectrum the violet most readily blackened the silver chloride. In Scheele, then, we have the first who applied combined chemical and spectrum analysis to the science of photo graphy. Senebier repeated Scheele s experiments, and found that in fifteen seconds the violet rays blackened silver chloride as much as the red rays did in twenty minutes. 1 About twenty years later than Scheele s experi ments Count Rumford contributed a paper to the Philo sophical Transactions of the Royal Society (1798) entitled " An inquiry concerning the chemical properties that have been attributed to light," in which he tried to demonstrate that all effects produced on metallic solutions could be brought about by a temperature somewhat less than that of boiling water. Robert Harrup in 1802, however, con clusively showed in Nicholson s Journal that, at all events, salts of mercury were reduced by visible radiation and not by change of temperature. In 1801 we come to the next decided step in the study of photographic action, when Ritter proved the existence of rays lying beyond the violet limit of the spectrum, and found that they had the power of blackening silver chloride. Such a discovery naturally gave a direction to the investigations of others, and See- beck (between 1802 and 1808) and Berard turned their attention to this particular subject, eliciting information which at the time was of a valuable nature. We need only mention two or three other cases where the influence of light was noticed at the beginning of this century. Wollaston observed the conversion of yellow gum guaiacum into a green tint by the violet rays, and the restoration of the colour by the red rays, both of which, be it observed, 1 It may here be remarked that had he used a pure spectrum he would have found that the red rays did not blacken the material in the slightest degree. are the effect of absorption of light, the original yellow colour of the gum absorbing the violet rays, whilst the green colour to which it is changed absorbs the red rays. Davy found that puce-coloured oxide of lead, when damp, became red in the red rays, whilst it blackened in the violet rays, and that the green oxide of mercury became red in the red rays, again an example of the necessity of ab sorption to effect a molecular or chemical change in a sub stance. Desmortiens in 1801 observed the change effected in Prussian blue, and Bockman noted the action of the two ends of the spectrum on phosphorus, a research which, it may be mentioned, Draper extended further in America at a later date. To England belongs the honour of first producing a Wedg- photograph by the utilization of Scheele s observations on chloride of silver. In June 1802 Wedgwood published in the Journal of the Royal Institution the paper " An account of a method of copying paintings upon glass and of making profiles by the agency of light upon nitrate of silver, with observations by H. Davy." He remarks that white paper or white leather moistened with a solution of nitrate of silver undergoes no change when kept in a dark place, but on being exposed to the daylight it speedily changes colour, and, after passing through various shades of grey and brown, becomes at length nearly black. The alteration of colour takes place more speedily in proportion as the light is more intense. "In the direct beam of the sun two or three minutes are sufficient to produce the full effect, in the shade several hours are required, and light transmitted through different-coloured glasses acts upon it with different degrees of intensity. Thus it is found that red rays, or the common sunbeams passed through red glass, have very little action upon it ; yellow and green are more efficacious, but blue and violet light produce the most decided and powerful effects. " Wedgwood then goes on to describe the method of using this prepared paper by throwing shadows on it, and infer- entially by what we now call "contact printing." He states that he has been unable to fix his prints, no wash ing being sufficient to eliminate the traces of the silver salt which occupied the unexposed or shaded portions. Davy in a note states that he has found that, though the images formed by an ordinary camera obscura were too faint to print out in the solar microscope, the images of small objects could easily be copied on such paper. "In comparing the effects produced by light upon muriate of silver (silver chloride) with those upon the nitrate it seemed evident that the muriate was the most susceptible, and both were more readily acted upon when moist than when dry a fact long ago known. Even in the twilight the colour of the moist muriate of silver, spread upon paper, slowly changed from white to faint