The machine in question is represented at Figures 15 and 16. Figure 15 is a plan, and Figure 16 a view of it on a larger scale. The same letters refer to the corresponding parts in both figures. A B E F is an oblong board, and x v are two hinges on which the part c r. n is moveable. This part con sists of two arches or portions of arches, c sn L, and n N L, joined together at the top L, and at the bottom to the cross bar D D, to which one part of each hinge is fixed, and the other part to a flat board, half the length of the board A B E F, and glued to its uppermost side. The centre of the arch c L is at n, and the centre of the arch D N L is at c.
On the outer side of the arch n N L is a sliding piece, s, (much like the nut of the quadrant of altitude belonging to a eommon globe) which may be moved to any part of the arch between n and L ; and there is such another slider o, on the arch c u L, which may be set to any part between c and L. A. thread c P N is stretched tight from the centre c to the slider N, and such another thread is stretched from the centre D to the slider o ; the ends of the threads being fastened to these centres and sliders. It is plain, therefore, that by moving the sliders on their respective arches, the intersection r of the threads may be brought to any point of the open space within those arches.
In the groove is a straight sliding bar 1, which may be drawn farther out, or pushed further in, at pleasure. To the outer end of this bar r, Figure 16, is fixed the upright piece n z, in which is a groove for receiving the sliding piece o. In this slider is a small hole a, for the eye to look through in using the machine ; and there is a long slit in n z, to let the hole a be seen through when the eye is placed behind it, at any height of the hole above the level of the bar 1.
Suppose a house, q s r p, to be at a considerable distance beyond the limits of the plate, to obtain a perspective repre sentation of it, place the machine on a table, with the end E F, of the horizontal board A B E F towards the house, so that, when the arch n L c is set upright, the middle part of the open space (about r) within it, may be even with the house when the eye is placed at z, and looking at the house through the small hole a ; and then fix the corners of a square piece of paper with four wafers, on the surface of that half of the horizontal board which is nearest the house.
To complete the arrangement of the apparatus for draw ing, set the arch upright, as in the figure, which it will be when it comes to the perpendicular side 'r, of the upright piece 5 T, fixed to the horizontal board behind D. Then placing the eve at z, look through the hole at a at any point of the house, as q ; and move the. sliders N and o, till the intersection of the threads. at r, is directly bet Nvcen the eye and the point ; then put down the arch flat upon the paper on the board, as s t, and the intersection of the threads will be at w. Mark the point w on the paper with the dot of a blacklead pent;i1, and set the arch upright again as before ; then look through the hole it, and move the sliders s and o, till the intersection of the threads comes between the eye and any other point of the house, as w ; this being done put down the arch again to the paper, and make a pencil-mark thereon at the inter section of the thread as before ; obtain the point p in the same manner, and draw a line from that mark to the one at w. The line p w, thus obtained, will be a representation in true perspective of the corner p q of the house.
By thus bringing the intersection of the threads success ively between the eye and other points of the outlines of the house, as r s, &c., and putting down the arch to mark the corresponding points, on the paper, at the intersection of the threads, then connecting these points by straight lines, the entire perspective outline of the house will be obtained. In like manner, find points for the corners of the doors, win dows, &c., and draw the finishing lines from point to point.. The perspective drawing thus produced, may then be com pleted, by shading it according to the manner in which the light is observed to fill1 on the original.
Great care must be taken, during the whole of the time, that the position of the machine be not shifted on the table; and to prevent such an accident, the table or support em ployed should he perfectly steady, and the machine fixed dew a upon it by screws or clamps.
It is obvious that a landscape, or any number of objects within the field of view through the arch, may be delineated, by finding a sufficient number of points, and connecting them by straight or curved lines, as they appear in the original objects.