PHOTOMETRY 4T3 so that as a general rule photographers prefer the wet silver process, as they can at once see what they are about, and if the negative is not good can make a second trial. Woodbury's photo-relief process consists in preparing a gel- atine print and transferring this to soft metal by a hydraulic press. The raised parts of the gelatine film are forced into the soft metal, thus giving a picture with the light and shad- ows reversed. A hot mixture of gelatine, with some coloring substance, is poured upon the intaglio plate, a sheet of well sized paper is then laid on, and the whole is pressed ; as soon as the gelatine is cool, the impression is re- moved and further fixed by alum and tannin. Osborne's photo-lithographic process is still an- other adaptation of bichromated gelatine. (See LITHOGRAPHY, vol. x., p. 529.) On the fine arts the effect of photography has been impor- tant, and every day is increasing the number of its applications to artistic purposes. Soon after the discovery of the collodion process its facilities for the multiplication of copies at- tracted the attention of publishers. Mr. G. P. Putnam of New York was the first to intro- duce it practically by the insertion of some photographic views in the " Homes of Ameri- can Authors" (New York, 1852). In France and Germany many splendid books have been illustrated by the same method, as the superb memorial edition of Schiller with photographs from the original drawings of eminent living artists of Germany. For the representation of scenery or the copying of old paintings, draw- ings, &c., it is equally available. The u Domes- day Book" has been perpetuated by photogra- phy, and Tischendorf 's great edition of the Co- dex Sinaiticus, one of the most ancient manu- scripts of the Bible, published at the expense of the emperor of Russia, contains facsimile pages of the original produced by photo-lithography. On the art of painting, the unlimited supply of photographic studies, combining breadth of effect with nature's own minuteness of detail, is producing the most marked results ; while, should the progress of photography keep pace with its past achievements, many of the labo- rious and expensive enterprises of engraving on steel and copper will soon be entirely su- perseded. See " The Amateur's Photographic Guide Book," by W. J. Stillman (London, 1874), and "The Chemistry of Light and Photog- raphy," by Hermann Vogel (English transla- tion," New York, 1875). PHOTOMETRY (Gr. 0a ? , light, and ptrpm>, measure), the process of measuring the inten- sity of light. The first quantitative compari- son of different sources of light with much approach to accuracy was made by Huygens about the middle of the 17th century. He used a tube having a small aperture at one end, in which was placed a minute globular lens which allowed the 27,664th part of the sun's disk to be seen. This fraction of his light being equal in brightness to the star Sirius, Huygens concluded that the distance of Sirius from the earth was 27,664 times as great as that of the sun. (Hugenii Cosmotheoros.) Bou- guer was led, by observations of Marain on the relative intensity of the sun's light at the sum- mer and winter solstices, to make an extensive series of investigations on the subject, and his results were published in his Essai tfoptique (1729). His photometer compared the reflect- ing powers of two different surfaces by having the image of one reflected in a mirror which was placed in a line with the other surface and the eye of the observer. The two re- flecting surfaces had a light placed between them, which was moved from one toward the other until the reflection of the one in the | mirror was equal in intensity to the direct light from the other; and as that intensity is in proportion to the square of the dis- tance, the reflecting or absorbing powers of the two surfaces could be readily computed by measuring the distance of the light from each of the surfaces. Lambert, ten years later, pub- lished his Photometria, a work of great math- ematical elegance. He describes a photome- ter by which the reflecting and transmitting powers of different translucent materials may be accurately ascertained for various angles of incidence. The photometer of Bouguer was modified by Ritchie so that a comparison of sources of light could be made, as well as the reflective powers of surfaces. A box with the axis placed horizontally has two plane reflec- tors, c b and e d, cut from the same plate, placed with two edges at right angles at c so that rays of light parallel with the axis of the box will be reflected perpendicularly upon the oiled pa- per screen at #, a blackened diaphragm being placed at a c. When the lights are placed so that the illumination on both sides of a is the same, the square of the distance of the Bouguer and Kitchie's Photometer. lights from the mirrors will give the compara- tive intensity. Rumford's photometer, valua- ble because it may be easily extemporized, em- ploys the principle of comparing the depth of shadows, and consists of a vertical staff placed a short distance in front of a screen of tissue or oiled paper. The shadows may be compared in front, or on the back side of the paper, the latter method being preferable because the back of the paper may be in a darkened chamber, thus allowing the eye to be undisturbed by the glare of the lights. Bunsen's photometer con- sists of a screen of thin writing paper stretched on a frame and marked with a grease spot. If a light is placed on each side of the paper, the spot viewed from the side of strongest light
