PERSPECTIVE MACHINE, is an instrument by which any person, without the help of the rules of art, may delineate the true perspective figures of objects. Mr Ferguson has described a machine of this sort of which he ascribes the invention to Dr Bevis.
Fig. 4. of Plate CCCLXXXVII. is a plane of this machine, and fig. 5. is a representation of it when made use of in drawing distant objects in perspective.
In fig. 4. abef is an oblong square board, represented by ABEF in fig. 5. x and y (X and Y) are two hinges on which the part cl/d (CLD) is moveable. This part consists of two arches or portions of circles cm/l (CML) and dn/l (DNL) joined together at the top l (L), and at bottom to the cross bar dc (DC), to which one part of each hinge is fixed, and the other part to a flat board, half the length of the board abef (ABEF), and glued to its uppermost side. The centre of the arch cm/l is at d, and the centre of the arch dn/l is at c.
On the outer side of the arch dn/l is a sliding piece n (much like the nut of the quadrant of altitude belonging to a common globe), which may be moved to any part of the arch between d and l; and there is such another slider o on the arch cm/l, which may be
set to any part between c and l.—A thread c p n (CPN) is stretched tight from the centre c (C) to the slider n (N), and such another thread is stretched from the centre d (D) to the slider o (O); the ends of the threads being fastened to these centres and sliders.
Now it is plain, that, by moving these sliders on their respective arches, the intersection p (P) of the threads may be brought to any point of the open space within the arches.—In the groove k (K) is a straight sliding bar i (I), which may be drawn further out, or pushed further in at pleasure.
To the outer end of this bar I (fig. 5.) is fixed the upright piece HZ, in which is a groove for receiving the sliding piece Q. In this slider is a small hole r for the eye to look through, in using the machine; and there is a long slit in HZ, to let the hole r be seen through when the eye is placed behind it, at any height of the hole above the level of the bar I.
How to delineate the perspective figure of any distant object, or objects, by means of this machine.
Suppose you wanted to delineate a perspective representation of the house qsrp (which we must imagine to be a great way off, without the limits of the plate), place the machine on a steady table, with the end EF of the horizontal board ABEF toward the house, so that, when the Gothic-like arch DLC is set upright, the middle part of the open space (about P) within it may be even with the house when you place your eye at Z and look at the house through the small hole r. 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; and all is ready for drawing.
Set the arch upright, as in the figure; which it will be when it comes to the perpendicular side r of the upright piece st fixed to the horizontal board behind D. Then place your eye at Z, and look through the hole r at any point of the house, as q, and move the sliders N and O till you bring the intersection of the threads at P directly between your eye and the point q; then put down the arch flat upon the paper on the board, as at ST, and the intersection of the threads will be at W. Mark the point W on the paper with the dot of a black lead pencil, and set the arch upright again as before; then look through the hole r, and move the sliders N and O till the intersection of the threads comes between your eye and any other point of the house, as p; then put down the arch again to the paper, and make a pencil mark thereon at the intersection of the threads, and draw a line from that mark to the former one at W; which line will be a true perspective representation of the corner pq of the house.
Proceed in the same manner, by bringing the intersection of the threads successively between your eye and other points of the outlines of the house, as r, s, &c. and put down the arch to mark the like points on the paper, at the intersection of the threads: then connect these points by straight lines, which will be the perspective outlines of the house. In like manner find points for the corners of the door and windows, top of the house, chimneys, &c. and draw the finishing lines from point to point: then shade the whole, making the lights and shades as you see them on the house itself, and you will have a true perspective figure of it.—Great care must be taken, during the whole time, that
the position of the machine be not shifted on the table; and to prevent such an inconvenience, the table should be very strong and steady, and the machine fixed to it either by screws or clamps.
In the same way, a landscape, or any number of objects within the field of view through the arch, may be delineated, by finding a sufficient number of perspective points on the paper, and connecting them by straight or curved lines as they appear to the eye. And as this makes every thing in perspective equally easy, without taking the trouble to learn any of the rules for drawing, the operations must be very pleasing and agreeable. Yet as science is still more so, we would by all means recommend it to our readers to learn the rules for drawing particular objects; and to draw landscapes by the eye, for which, we believe, no perspective rules can be given. And although any thing may be very truly drawn in perspective by means of this machine, it cannot be said that there is the least degree of science in going that way to work.
The arch ought to be at least a foot wide at bottom, that the eye at Z may have a large field of view through it; and the eye should then be, at least, 10½ inches from the intersection of the threads at P when the arch is set upright. For if it be nearer, the boundaries of view at the sides near the foot of the arch will subtend an angle at Z of more than 60 degrees, which will not only strain the eye, but will also cause the outermost parts of the drawing to have a disagreeable appearance.—To avoid this, it will be proper to draw back the sliding bar I, till Z be 14½ inches distant from P; and then the whole field of view, through the foot wide arch, will not subtend an angle to the eye at Z of more than 45 degrees; which will give a more easy and pleasant view, not only of all the objects themselves, but also of their representations on the paper whereon they are delineated. So that, whatever the width of the arch be, the distance of the eye from it should be in this proportion: As 12 is to the width of the arch, so is 14½ to the distance of the eye (at Z) from it.
If a pane of glass, laid over with gum water, be fixed into the arch, and set upright when dry, a person who looks through the hole P may delineate the objects upon the glass which he sees at a distance through and beyond it, and then transfer the delineation to a paper put upon the glass, as mentioned in the beginning of the article PERSPECTIVE.
Mr Peacock likewise invented three simple instruments for drawing architecture and machinery in perspective, of which the reader will find sketches and descriptions in the 75th volume of the Philosophical Transactions. We do not insert these descriptions here, because we do not think the instruments superior to that described by Ferguson, and because we wish that our readers who have occasion to draw may make themselves so much masters of the art of perspective, as to be above the aid of such mechanical contrivances.