Page:The American Cyclopædia (1879) Volume III.djvu/659

 CAMERA LUCIDA 653 moving from the block prior to mounting." Oameo8 carved in onyx and carnelian demand more skill, as well as labor, tban those In shell. A drawing is first made on an enlarged scale, and from this a model in wax of the exact size. The outline is then drawn on the stone, and the engraving is executed with the tools used by the lapidary for engraving seals, being drills of soft metal, as copper or iron, made to re- volve rapidly, and fed with emery and oil. False cameos are sometimes made by carefully cutting out the engraved portion of antique gems and attaching this to a ground of agate of another color. Beudant refers to some cameos in a slaty kind of onyx, schistes onyx, which are brought from China as objects of curiosity. They are sheets of rock resembling very compact slates, and presenting three or four differently colored layers; one a brown, which is the ground, others red, white, and greenish. In these the Chinese have sculp- tured various objects, as the interiors of houses, and landscapes, which are sometimes enlivened with figures of men and animals. Some are so large that they may be regarded as bass re- liefs for interior decorations. The art of cut- ting cameos in onyx and sardonyx, which for gome years has been comparatively neglect- ed, or has been superseded by the cheaper and more easily wrought shell cameos, has recently been revived in Rome and in Paris, though the best productions of these cities are inferior to the work of ancient and mediaeval artists. In England the art has never been sufficiently en- couraged to induce many artists to pursue it exclusively, though the work of some of the London gem engravers bears a high reputation. Paris is now the chief centre of cameo cutting, and there are to he found the best artists, designs, and facilities for the work. The Franco-Ger- man war of 1870 drove out many of these ar- tists, five or six of whom emigrated to New York, where cameo cutting has been carried on to a limited extent for several years, and where there are a few artists who execute in shell portraits and other designs for rings and brooches which are creditable as works of art. But cameo cutting, as a successful business in the United States, is only beginning to exist, and that almost exclusively in New York. A single jewelry house in that city had orders in 1872 for 5,000 cameos, such as ring stones worth from $2 to $50 each, and brooches from $20 to $250 each ; and all these orders were executed in Paris, excepting perhaps 50 stones, and these mainly to complete imported sets, which were cut in New York. CAMERA I.I ( I IH. an instrument invented by Dr. Wollaston, and constructed on the prin- ciple that when a beam of light in passing through a glass prism strikes an interior sur- face at an incident angle of more than 48 30' it is totally reflected. Let A B C D, fig. 1, represent a transverse section of a prism having the sides A D and D C at right angles, the angles A and C each 67 30', and the angle at B 135. If now a ray of light enter the prism from 6 perpendicular to the face D C, and of course parallel with A D, it will pass without refraction to the point a, where it will Fio. 1. be totally reflected in consequence of the angle of incidence bad being greater than 41 48'. In this case the angle of incidence will be 67 30', so that the sum of the angles of incidence and of reflection, b a c, will be 135, and equal to the angle ABC. The reflected ray a c will therefore strike the face A B at c with an angle of incidence of 67 30', and suffer reflec- tion in the line c d perpendicular to the face A D (360 4 x 67 30'=:90). If the eye could be placed anywhere on the line a c, it would perceive an inverted image at a of an object at & ; but when the eye is placed at d, an erect image is seen in consequence of the two reflec- tions. But as the instrument is intended as an aid in drawing, it is necessary to have an image of the object projected on a sheet of paper. This is effected by placing the eye so near the edge of the prism that the image and the paper may be seen at the same time, as in fig. 2. In consequence, however, of the image Fio. 2. appearing further from the eye than the paper, the use of the instrument is attended with considerable practical difficulty, which is only partially overcome by placing a convex lens between the eye and the prism. Few persons ever succeed in obtaining satisfactory results, while some acquire considerable facility of ma- nipulation. Another form, devised by Amici, is more manageable than Wollaston's, in con- sequence of not confining the eye to one par- ticular place. A right-angled triangular prism, in place of a quadrilateral one, is employed, which produces two refractions and one reflec- tion. This is represented in fig. 3. The prism is so placed that one of its sides bounding the right angle is perpendicular to a plate of glass which is used as a reflector. Those rays of