A mechanical contrivance designed to facilitate the making of drawings in perspective, especially by such persons who are unacquainted with the rules by which it is performed. Some of these instruments are on optical principles, such as the camera obscura, and the camera lucida, already noticed under their proper heads. In praise of the latter much has been lately said, and although it must be admitted to be a very portable and beautiful instrument, the acquisition of the art of using it is extremely difficult to all, and to some persons impossible. Its chief use will be that of affording the means of contemplating the real perspective appearance of objects, and perhaps to obtain the position of a few points, but for very minute delineation it is of little value. One of the simplest mechanical contrivances for taking successively on the perspective plane the various points of an object or landscape, and marking them down on paper with accuracy, was long since described by Ferguson, to whom the knowledge of it was communicated by Dr. Bevis. It consisted of an oblong rectangular board, across the middle of which was attached by hinges a movable frame, the sides of which were formed of two equal circular arcs, that met together at the top in the manner of a gothic arch.
To the centre of each of these arcs was attached a cord, the other ends of which were fastened to sliding pieces traversing their respective arcs; these cords therefore crossed each other, and by moving the slides to any part of the opposite arcs, the cords might be made to intersect each other at any point in the plane, or space between the arcs. The eye-piece, or hole through which the object to be drawn is viewed, is fixed to a slide in the centre of one end of the board, and the distance between the eye and the plane of delineation may be thus varied to increase or diminish the size of the picture. On that half of the board between the frame, (when the same is turned upright on its hinges,) and the object viewed, the paper to be drawn upon is pinned down. It will now, we think, be plain, that to mark down the exact position of any point in a picture, it is only necessary to move the slides so that the cords shall intersect at that point; having thus found it, the arched frame is then turned down upon its hinges flat upon the board or paper thereon, a mark is then made on the latter at the point of intersection of the cords.
Suppose the mark thus made indicates the extremity of the parapet of a building, the slides being then moved so as to intersect the cords at the other extremity, that point is also found, and the frame again turned down to mark it; then by connecting these two points on the paper by a line, the precise inclination and perspective measurement is infallibly correct, and by proceeding to work in this manner, all the outlines of an entire picture may be accurately laid down.
Many instruments have been contrived for finding the various perspective points, but the process, it must be allowed, is extremely slow; even the most simple figure would require to have many points found in it before its outline can be produced; and if it consisted of curved or irregular lines, many more points must be taken in each curve to get a correct delineation.
Mr. F. Ronalds, of Croydon, has, however, contrived and patented an apparatus, by which the lines themselves, of whatever form or arrangement they may be, maybe drawn directly from the object with the same facility as tracing them. The operation simply consists in causing a small bead to traverse in the plane of delineation, but the bead cannot make any movement whatever without a pencil mechanically attached to it, which traces down on paper, lines precisely corresponding with the figure; in other words, while the bead traverses over the lines of the object, the pencil moving with it does of necessity make an accurate perspective drawing. Fig. 2, in the following page, gives a perspective view of one of the forms of the complete instrument in the manner it is used, except that the legs on which it stands are cut off to save space. But in order that the reader may easily comprehend its action, we subjoin the annexed diagram, illustrative of the principle. The instrument consists of a straight bar ee, moving horizontally on two rollers attached to the table; f l are two other bars fixed at right angles to the bar e e, and to each other, the former lying on the drawing paper (horizontally), the latter placed perpendicularly in the plane of the picture, all being attached together: if the barf be moved to the right or left, the vertical rod l will slide on the rollers in a vertical plane, or the plane of delineation.
To the barfis adapted a slider with a pencil, as seen in Fig. 2; to this pencil a silk thread is fastened, which passes under a pulley in the corner where all the bars meet; thence it proceeds upwards, parallel to the bar l, (at which part it carries the small bead,) and finally passes over a pulley at the top, having a little weight which falls down the bar or tube l attached to its other end.
It will now be evident, that if we move the slider with the pencil on the horizontal bar, the weight attached to it by means of the silk thread must rise or fall through an equal space, and with it the bead placed upon it; and whether the pencil be moved to the right or left, or along the barf, the bead must move in the same direction, but in a plane at right angles to it. Having explained these two motions, it follows that every combination of them, whether in curved or straight lines, must be similarly performed both by the bead and pencil.
In using the instrument, it is requisite to arrange the sight-hole, attached to the bar T, (through which alone the operator must use his eye in sketching,) and the position of the bead on the thread, so as to get the drawing within the limits of the drawing paper. The handle which is attached to the slider with the pencil by an universal joint, must now be moved about, causing the bead to traverse over every line of the object, which, being marked down by the pencil, we have a fac-simile of the motions of the bead on the plane of delineation. We have in this most ingenious instrument a simple and elegant adaptation of the foundation laws of the science of perspective; it may be called a teacher of perspective as well as a perspectograph. These instruments are constructed of various sizes, and packed in cases, including a book of instructions, at very moderate charges. They are manufactured by Messrs. Holtzapf'ell, of Charing Cross, in the best style of workmanship.