Further experiments and observations, setting forth the advantages that electricians are likely to obtain from the above discovery, will be the subject of a future paper.

No. 39.

Description of the Optigraph (invented by the late Mr. RAMSDEN) as improved and made by Mr. THOMAS JONES.*

(With an engraving.)

THE methods used to facilitate the practice of drawing in perspective, as well for those versed in this polite art, as for those who have made less proficiency, have been various and numerous. Though some have supposed that the warmth of imagination and luxuriance of fancy, which impel the mind to the cultivation of the fine arts, are not to be confined to mechanical modes, yet upon observation and inquiry they will find that the most able and accomplished artists are often obliged to have recourse to some rules, and to use some mechanical modes to guide and correct their pencil: but so tedious is the operation, and great the difficulty, of representing objects in true perspective, that they trust mostly to their eye and experience for success. The result of such a mode of proceeding may be determined by portraits drawn by the best artists, and the different judg ments formed concerning them. It has been well observed, that there is no artist who will be hardy enough to say that he can delineate by the eye the same object twice with exactness, and preserve a just and similar proportion of parts in each. In one of the figures we

shall find some of the parts larger than in the other; both cannot be right: yet supposing them perfectly the same, neither may be conformable to nature. In addition to this, many situations of an object occur, which no eye, however habituated, can represent with accuracy.

On this account many attempts and various instruments have been made for the purpose of giving the outline of an object with accuracy.

The late most ingenious Mr. Ramsden, so well known for his inventions and improvements in various instruments, considered the present subject an object worthy of his attention, and invented the instrument I am about to describe, which is so simple and easy in its operation, that a person not possessed of the least knowledge of drawing, may, with less than three minutes' instruction, be perfectly able to take a perspective view of landscape, building, machinery, or, in fact, an object of any description presented to his eye, with the utmost correct

ness.

Mr. Ramsden left this instrument without the means of enabling the operator to enlarge or diminish his drawing; an inconvenience which I have obviated, while at the same time I have added some other trifling improvements. This instrument is certainly superior to any hitherto constructed for the same purpose; for in this the operator views the object through a telescope, which enables him to delineate minute objects with great exactness and ease, which are often too far from the eye to be seen sufficiently well to be delineated correctly.

Fig. 3, (plate 6,) is a perspective view of the optigraph. A represents the drawing board, on the outside frame of which is fixed the pillar of the instrument, B, by a clamp a. C is a tube, (sliding in the pillar,) on the top of which is fixed, by means of a screw c, the frame

D; at the end of this frame is a plain mirror E, beneath which is suspended, by a universal joint, the telescope F, of which G is the eye-tube. H are sliding tubes, capable of being shortened or lengthened in the same proportion as the inside speculum c, (fig. 4,) which is fixed to any place by the clamp screw P. The pencil L, of which h is the handle, slides perfectly easy, without shake, in the tubes H: the pencil is so contrived as to have all the freedom of a pen when held in the hand for use.

Fig. 4, represents a section of the telescope, being the principal part of the invention. The rays from an object entering the plain mirror a, are reflected into the telescope, passing through the object-glass b, and entering the speculum c, are reflected through the eye-glass d, to the eye at e: ƒ is a piece of parallel glass, with a small dot on its centre, exactly in the focus of the eyeglass d.

Mode of Using the Optigraph.

Fix the drawing board to the table (by a clamp which is packed in the box) so that the surface of the mirror E is nearly parallel to the object; then take hold of the handle h, and hold the pencil on that part of the paper where you would wish the centre of your drawing, or any part thereof, to be. Then place your eye at the eye-tube G, and with your left hand alter the inclination of the mirror E until the small dot, described at f, in fig. 4, is on some particular part of the object that you wish to begin with, adjusting the telescope to distinct vision by the milled head P. Then by moving your hand (having the pencil) you pass the dot seen in the field of the telescope over the object, the pencil marking it at the same time on the paper.

To make your drawing larger, pull out the tube of the pillar C, fig. 3, and fix it with the screw e; then pull out the sliding tubes H, till the pencil is within half an inch of the paper, (in the middle of the board,) and proceed as before.

To make the drawing smaller, shorten the tubes C and H by sliding them in, and proceed as before.

No. 40.

Description of an Instrument for drawing in true Perspective from Nature, and of another of considerable Simplicity and Cheapness for delineating Ovals. By R. B.*

To Mr. Nicholson.

[With an engraving.)

SIR-As I observe that you are willing, in your capacity of Journalist, to lay before the public any sketch or outline of invention that may promise to be useful, whether in its ultimate state of improvement or not, I am encouraged now and then to send my thoughts, queries, observations, or news, as they may occur. The following instruments are offered to your notice, in hopes they may appear in your excellent collection.

Fig. 1, Plate 6, is a sketch of an instrument for perspective, made some years ago by Dolland, and of which I know not the inventor. A telescope or camera is sus pended vertically on a frame by an universal joint or jimbals. Horizontal rays A, are directed down the tube by a plane mirror B, and are again rendered horizontal,

and turned to the eye through a side hole in the tube, by another mirror C. At the lower end is a pencil E sliding in a well-fitted socket, and pressed gently downwards by a weight or spring; or still better, by the hand only. The result or use is, that while the images are in succession brought into apparent contact with a point in the field of view, the pencil may be employed in tracing them in true perspective upon the table beneath.*

Fig. 2, represents a simple rule and string for drawing ovals on paper. A CB is a silken thread, fixed at A, and capable of being lengthened, shortened, and fixed by a screw B at the other end. This screw B can be placed, by a longitudinal groove in the ruler, at any distance from A, and can be made to pinch the thread upon any one of the divisions of the rule. At C is a pencil to be moved in the bend of the thread. It must be held upright, and it would be easy to contrive means of keeping it so; but it does not seem an object of sufficient necessity to add to the price of the instrument.

In the use, set A at one focus of the intended oval and Bat the other. Allow the string to extend till the pencil marks the extremity of the conjugate diameter. Draw the semi-oval by moving the pencil along in the stretched thread: Then reverse the points A and B, placing them respectively on the foci occupied before by each other. Draw the other semi-oval, which completes the figure. I am, &c.

R. B.

There is an omission of the grey or rough glass, if the drawing be meant for a camera; or of the eye-piece, if it be a telescope. The first focal convergence must be made in thesc, and not at the eye.-NICHOLSON.