Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/816

740 largely imported into England and mounted as brooches by Birmingham jewellery manufacturers. The principal shell used is the large bull s-mouth shell (Cassis rufa), found in East Indian seas, which has a sard-like underlayer. The black helmet (Cassis tuberosa] of the West Indian seas, the horned helmet (C cornuta) of Madagascar, and the pinky queen s conch (Strombus gigas) of the West Indies are also employed. The famous potter Josiah Wedgwood introduced a method of making imitations of cameos in pottery by producing white figures on a coloured ground, this con stituting the peculiarity of what is now known as Wedgwood ware.  CAMERA LUCIDA, an instrument invented by Dr Wollaston for drawing in perspective. If a piece of plane glass be fixed at an angle of 45 with the horizon, and if, at some distance beneath, a sheet of paper be laid horizontally on a table, a person looking downwards through the glass will see an image of the objects situated before him ; and as the glass which reflects the image is also transparent, the paper and pencil can be seen at the same time with the image, so that the outline of the image may be traced on the paper. The image is an inverted one. This is the simplest form of the instru ment, and may be constructed extemporaneously by fixing on a stand a plane transparent glass, with its surfaces ground parallel, or a piece of Muscovy glass, at an angle of 45 with the horizon. A card with a small hole in it will serve as a sight for keeping the eye steady in one situation whilst the pencil is tracing the image. Let a plane mirror, cb (Plate XXXIV. fig. 1), be inclined at an angle of 22/ with the horizon, and let ba, a piece of plane transparent glass, be so placed as to make an angle of 22J with the vertical, then rays/&amp;lt;/ from an object will be twice reflected before they reach the eye at e, and, con sequently, on looking down through the transparent glass, an erect image is seen, and the pencil may be drawn over the outlines of this image, so as to leave a perspective representation on the paper. As the image and pencil are at different distances, they cannot be both seen in the same state of the eye. To remedy this inconvenience, a convex glass is used, of such focus as to require no more effort than is necessary for seeing the distant objects distinctly. By means of this lens, the image will appear as if it were placed on the surface of the paper. In fig. 1, bd is a convex glass of 12 inches focus. Instead of using a convex lens, short-sighted persons will require a concave glass to be placed at /, in the course of the rays from the object to the reflecting surface. In fig. 2, ik is a concave glass so placed that it may be turned at pleasure into its place, as the sight of the observer may require. Persons whose sight is nearly perfect may use either the concave glass placed before the reflecting surface, or the convex placed between the paper and the eye. In the actual construction of the instrument, a prism is used instead of a mirror and plane glass. The rays from the object fall upon the surface be of the prism, fig. 3. This surface be is inclined 22/ to the horizon. The refractive power of the glass allows none of the rays in this situation to pass out ; they are all reflected from the surface be to the surface ab, and from that to the eye. ab makes an angle of 135 with be, and 22i with the vertical. The eye cannot see the pencil through the prism as it does through a plane glass ; therefore, in order that the pencil may be seen, the eye must be so placed that only a part of the pupil may be above the edge of the prism, as at e, fig. 3 ; and then the reflected image will be seen at the same time with the paper and pencil. There is a small piece of brass perforated with a hole c (fig. 2), and moving on a centre ; this serves to keep the eye in one position, as it must be that the image may be steady, and also to regulate the relative quantities of light to be received from the object and from the paper. The instrument, being near the eye, does not require to be large. The smallest size which can be executed with accuracy is to be preferred, and is such that the lens is only three-fourths of an inch in diameter. Fig. 4 shows the instrument on its stand, and clamped to a board. The joint by which the prism is attached to the stand is double. This instrument serves for drawing objests of all forms, and consequently also for copying lines already drawn on a plane surface. If it is required that the copy shall be of the same size as the original drawing, the distance of the drawing from the prism should be the same as the distance of the paper from the eye-hole. No lens will be necessary in this case, because the image and the paper, being both at the same distance from the eye, coincide without the aid of a glass. In order to have a reduced copy of a drawing, the drawing is to be placed at a distance from the prism greater than the distanca of the paper from the eye-hole. If the distance is twice as great, a copy will be obtained in which the lines are of one-half the size of the lines in the original, and so in proportion for other distances. A lens is necessary, that the eye may be enabled to see at two different distances ; and, in order that one lens may serve, the distance between the eye-hole and the paper should be variable ; to that effect the stand is susceptible of being lengthened or shortened at pleasure. The length of the stem is adjusted upon optical principles. When a distant object is to be delineated, the rays coining from it, and reflected by the instrument to the eye, are parallel, and it is required that the rays proceeding from the paper to the eye should also be parallel. This is accomplished by interposing a lens between the paper and the eye, with its principal focus on the paper. When the object to be delineated is so near that the rays which come from it to the eye are divergent, then it is required that the rays from the paper should likewise be divergent in the same degree, in order that the paper and the image may both be seen distinctly by the same eye ; for this purpose the lens must be placed at a distance from the paper less than the distance of its principal focus. The advantage of this instrument as compared with the camera obscura are, 1st, That it is small and easily carried about ; 2dly, That no lines are distorted, not even those most remote from the centre ; and 3dly, That in the field of the camera lucida 70 or 80 may be included, whilst the distinct field of the camera obscura does not extend beyond 30 or 35 at most. See Repertory of Arts, vol. x., 1807, p. 162, and Nicholson s Journal, vol. xvii. If the camera lucida be fixed at the eye-glass of a telescope, it will reflect to the eye the image 6f the objects in the field of the telescope, so that a drawing of the image may be made. See Brewster s Account of some Philosophical Instruments. A plane reflecting glass fixed at an angle of 45 with the horizon, and placed so as to receive the rays from the eye-glass of a telescope, will also give an image of the objects in the field, so situated that the image may be traced with a pencil. Varley s patent graphic telescope is upon this principle. In order that the field may be large, the magnifying power of the telescope should be small. The optigraph of Ramsden and Thomas Jones, described in the Philosophical Magazine, vol. xxviii., 1807, p. 67, is an instrument of a similar kind. The image of the object is seen in a telescope. There is a piece of plane glass near c, in the focus of the eye-glass of the telescope F, Plate XXXIV. fig. 5. On the centre of this piece of glass is a dot; a is a plane mirror, inclined so as to reflect the image of the object into the telescope. This mirror remains fixed, whilst 