Lucida.

Camera be placed at f, in the course of the rays from the object to the reflecting surface. In fig. 2. Plate XLVI. ik is a concave glass placed in the above-mentioned situation; it is so disposed as to 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 glass placed between the paper and the eye.

Usual form

mera Lucida.

In the actual construction of the instrument, a

of the Ca- prism is used instead of a mirror and a plain glass. The rays from the object fall upon the surface bc of the prism fig. 2. This surface bc is inclined 22 degrees 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 bc to the surface ab, and from that to the eye; ab makes an angle of 135 degrees with bc, and 221 degrees with the vertical. The eye cannot see the pencil through the prism as it does through a plain 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, and moving on a centre at e, fig. 2.; 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.

Its Uses.

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. The whole instrument packs in a box eight inches by two, and half an inch deep.

This instrument serves for drawing objects of all forms, and consequently also for copying lines already drawn on a plain surface. If it is required that the copy shall be of the same size as the original drawing, the distance from 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 distance 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 onehalf 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 plea

sure.

The length of the stem is adjusted upon optical principles. When a distant object is to be delineat

ed, the rays coming from it, and reflected by the in- Camer strument to the eye, are parallel, and it is required Lucida 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 stem of the instrument is marked at certain distances, to which the conjugate foci are in the several proportions of 2, 3, 4, &c. to 1, so that distinct vision may be obtained in all cases by placing the original drawing more dis tant.

If the convex lens be transposed to the front of the prism, and the proportional distances be reversed, a magnified image of the object will be obtained.

Obse

This instrument has deservedly come into use. Its Compars advantages when compared with the camera obscura with the are, 1st, That it is small and easily carried about. Camera 2dly, That no lines are distorted, not even those most remote from the centre, whereas, in the camera obscura, the lines which are not near the centre of the field are more or less distorted. 3dly, In the field of the camera lucida 70 or 80 degrees may be included, whilst the distinct field of the camera obscura does not extend beyond 30 or 35 degrees at most. The specification of Dr Wollaston's patent for the camera lucida is inserted in the Repertory of Arts, Vol. X. 1807, p. 162, and his description of the instrument in Nicholson's Journal, Vol. XVII.

If the camera lucida be fixed at the eye-glass of a Its Appli telescope, it will reflect to the eye the image of the tin t objects in the field of the telescope, so that a drawing Telesc of the image may be made. See Dr Brewster's Account of some Philosophical Instruments. A plain reflecting glass, fixed at an angle of 45 degrees 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.

making

The inherent qualities of all the instruments for Varions drawing in perspective being closely allied, it will Modes be proper to say something of the principles on Drawing which these instruments are formed, and to men an Object tion some that are not described in other parts of this work.

To make a perspective drawing of an object is to lay down on paper a section of the perspective cone, whose apex is at the eye, and whose base is the object. An experienced draughtsman can draw the figure of this section without the aid of instruments. Others who have not acquired the facility of drawing the image they see, must have recourse either to measurement, or to the instruments, which bring the image under the pencil.

Camera

Drawing by measurement is performed by actually Lucida. measuring the height of the principal parts of the object, and their horizontal distance from the eye, together with the distance of the paper from the eye, and from these dimensions the drawing is constructed by the systematic rules of perspective.

Another mode of obtaining a drawing by measurement, is to measure the angles at the eye. Suited to this purpose are theodolites, astronomical quadrants, or other instruments, capable of measuring vertical and azimuthal angles at the eye. The angles to be measured are, the angles of altitude, and the angles of azimuth, between the point of sight and the principal points of the object; and if the tangents of the azimuthal angles be laid down with a radius equal to the distance of the paper from the eye, and the tangents of the angles of altitude with a radius equal to the distance of the paper multiplied by the secant of the azimuth, the situation of the principal points of the drawing will be determined. Or, if the instrument is capable of measuring angles in any plane, the angles between the principal points of the object and the point of sight are to be observed, and the azimuthal angles of these principal points with the point of sight; and the tangents of both are to be laid down on the paper, with a radius equal to the distance of the paper from the eye.

But these two modes by measurement are long, particularly the first. Usually, therefore, the instruments to which recourse is had for facilitating the operation of drawing, are such as give an image or section of the perspective cone on a plain surface, so that the pencil may be drawn over the outline of the image. These instruments may be considered under two heads. The first comprehending those in which the pencil is immediately drawn over the lines of the image. The second those in which the pencil has a motion parallel to that of the point which moves over the lines of the image.

Of the first kind are the following. 1. The tracing pane, a very simple and convenient instrument, consisting in a transparent plate of plain ground glass, or of Muscovy glass, placed vertically between the object and the eye; whilst the eye is kept fixed by a sight, the outline of the image is drawn on the glass with Indian ink. 2. Or the upright glass may be divided into small squares by lines crossing each other, and the paper on which the drawing is to be made, being similarly divided, the particular intersections on the glass that cover the principal points of the object are observed, and these points are laid down on the corresponding intersections on the paper. 3. The image seen in a plain mirror, may also be drawn on its surface with Indian ink. 4. In the camera obscura, different forms of which are described in the Encyclopædia under the articles DIOPTRICS and OPTICS, the image to be drawn is formed at the focus of a lens. 5. In the camera lucida the reflected image is used.

In the second division of the instruments which give a section of the perspective cone susceptible of being delineated, the pencil does not move immediately over the lines of the image, but moves parallel

to these lines. 1. There is a rod which can be moved in all directions, consistent with its remaining parallel to itself. If one extremity of this rod be moved in space over the outlines of the image which the eye sees, a pencil at the other extremity will necessarily move with a similar motion, and form a drawing of the object on paper, In Sir Christopher Wren's instrument, of which he has given the description and figure in the Philosophical Transac tions, Vol. IV., the rod is suspended by strings passing over pullies, and the ends of the strings are fixed to a counterpoise. On a similar principle is Peacock's instrument, described in the Philosophical Transactions, Vol. LXXV., p. 366; and the instruments treated of in the Stockholm Transactions for the years 1760, 1774, and 1790. 2. The pencil may delineate the base of a cone similar and opposite to the perspective cone. If the rays from the extreme points of an object cross on the ray from the centre, as they do in passing through a small hole into a dark room, and if it be supposed, that in the place of one of the rays a slender inflexible rod is substituted, moveable on a centre at the hole, when this rod is moved, so that its outer extremity goes over the outlines of the external image, a pencil fixed to its inner extremity will form an inverted drawing of the object. Of this nature is the optigraph of Ramsden and Thomas Jones, described in the Philosophical Magazine, Vol. XXVIII. 1807, p. 67. The image of the object is seen in a telescope. There is a piece of plain glass near c in the focus of the eye-glass of the telescope F, Plate XLVI., fig. 5. On the centre of this piece of glass is a dot: a is a plain mirror, inclined so as to reflect the image of the object down into the telescope; this mirror remains fixed, whilst the telescope is moveable on a universal joint at its objectglass b. Near c is another plain mirror, which reflects the rays to the eye-glass. The eye being placed at the eye-glass at e, the telescope is to be moved by the handle h, so that the dot in the focus of the eyeglass shall pass over the outlines of the image seen by the eye, and the pencil at L performing a similar motion to that of the dot, and sliding freely in its sheath, presses with its weight on the paper: a drawing of the object is the result. If the stand and slider H be lengthened, an enlarged drawing will be obtained. The instrument packs in a box 14 inches by 6 and 3.

(Y.)

CAMPER (PETER), eminent for his extensive knowledge in the various branches of Medicine, Zoology, and Comparative Anatomy, and for his taste in the Fine Arts, was born at Leyden, May the 11th, 1722. His family had long held distinguished situations in the magistracy of that city, where his grandfather had exercised the profession of medi. cine. His father, Florent Camper, was a Protestant clergyman, and had officiated in that capacity for some years at Batavia; but had returned to his native country in 1713, after marrying Sarah Ketting, who was born of Dutch parents, at Surat. Florent Camper was an enthusiastic admirer of painting, and took great delight in the society of artists, whom he treated with the greatest liberality,-his purse being

Camper. ever open to such as needed his assistance. He was much connected with the learned men who adorned the University of Leyden at the beginning of the last century; and was on terms of intimate friendship with the great Boerhaave. Young Camper has, no doubt, been greatly indebted for his success to the fortunate circumstances in which he was placed in early life; being thus surrounded by men of enlarg. ed and cultivated understandings, eminent for their taste and learning, and having, at the same time, every incentive that a wise education could supply to emulate those excellent models; but nature had besides endowed him with that inherent desire of knowledge, that capacity, and that vigour and activity of mind, which, united as they were with a robust constitution of body, enabled him to reap the full benefit of these advantages. He gave very early proofs of his possessing those mental qualities, which lay the foundation of future eminence; and his father, discerning with delight the auspicious dawn of his genius, judiciously removed whatever might cramp its growth, and avoided im posing on him as a task those instructions and attainments, which he seemed so well inclined to acquire and pursue as an amusement.

He applied himself, at an early age, to drawing and painting, under the tuition of Moor and of his son, both of whom were celebrated artists, and soon became remarkably proficient in these accomplishments. He derived, in the course of his life, immense advantage from the skill with which he used his pencil in delineating any object in which he was interested, whether among the works of art, or the productions of nature, or whether they were the offsprings of his own conception, in the course of his philosophical researches. The value of this acquirement, as an object of early education, is not, perhaps, in general, sufficiently appreciated. The power of conceiving readily, and with correctness, mechanical forms, is one of the most useful results that practice, in the delineation of objects, can confer, and is of incalculable advantage in a variety of pursuits, with which such a talent might not, at first sight, seem to be immediately concerned. There is no doubt, for instance, that it must remove many difficulties in the study of Geometry, by facilitating the conception of figured space, the properties of which are the subjects of that science. An accurate knowledge of Anatomy is still more directly dependant upon the same power of apprehending the relations of form. The progress which Camper made in this branch of science, and the range of inquiries to which he afterwards made his knowledge subservient, are striking illustrations of this position.

He was indebted to Labordes for his first lessons in Geometry, and was instructed in Natural Philo sophy by Musschenbrock and Gravesande, who were the intimate friends of his father, and whose names will be ever illustrious in the annals of science. From these studies he was naturally led to the pursuit of Medicine, of which the elementary branches have so close an alliance with the physical sciences; and having entered the university of Leyden, became the pupil of Gaubius, Van Rooyen, and the elder

Albinus; for Boerhaave was, by this time, incapaci- Cempe tated by the infirmities of age from continuing his exertions as public teacher in the university. Čamper earned the first fruits of his academical labours by receiving, in 1746, the degree of Doctor in Philosophy and Medicine; on which occasion he published two dissertations; the one De Visu, the other De oculi quibusdam partibus, which are mentioned with commendation by Baldinger, in his Biography of Living Physicians, and which have been preserved by Haller. In the former he illustrates and defends Smith's Theory of Vision, and in the latter describes and gives plates of Petit's Canal in the Eyes of different Animals.

The acquaintance he had formed at college with several foreigners of merit, had long inspired him. with a desire of travelling, and of gratifying his thirst for knowledge, by visiting different countries, and conversing with men distinguished for their ac quirements in the several branches of science. But the declining health of his parents, who were now advanced in years, and required the continual presence and kindest attentions of their son, long prevented him from accomplishing his wishes. Their death, however, which happened in 1748, released him from duties, which he had the consolation of reflecting had been so piously discharged; and he soon after, at the age of twenty-six, embarked for England. In London he met with the celebrated physicians Mead, Pringle, and Pitcairn, with whom he became acquainted. He pursued his medical studies under Hunter, Sharp, Smellie, and Winchester, and indulged his taste for natural history, by examining diligently the cabinets of Hans Sloane and Collinson, and the collections of Hill and Catesby. He studied Botany under Elliot, and Astronomy under Short; and was instructed in the use of the microscope by Baker, who was at that time applying this instrument with so much success to objects of natural history. He seemed determined to suffer no opportunity of amassing a store of useful knowledge to escape him; and although his views embraced a wide range of subjects, he was never satisfied with a superficial glance, nor trusted to the reports of others, when there was a possibility of seeing with his own eyes the objects of his curiosity. His attention was par ticularly directed to the mechanical arts; he visited the principal manufactories, and was indefatigable in collecting instructions from artists of eminence in every department; and his eager curiosity even extended to the details of naval architecture, to the study of which he devoted a considerable portion of time. He was in the habit, during all his travels, of making minutes of every thing he saw and learn. ed; and his happy facility in the employment of the pencil, enabled him to take sketches on the spot of every object of which a delineation could be useful. Knowledge thus derived from personal observation is the more valuable, as it is more strongly impress ed on the memory, and as it is less liable to inac curacy, and less likely to be tinctured with preju dice, or distorted by the medium through which it is received. He still cultivated his taste for painting, and acquired much practical skill in the art