CAMERA LUC IDA. the name of an instrument for enabling those who have no knowledge of the art of drawing, to take sketches either of natural or artificial objects. This ingenious and useful contrivance was in vented in the year i S07. by W. H. Wollaston. D. Sec R. S., a philosopher to whom science is indebted for nu merous inventions and discoveries. As this instrument is of extensive use, its ingenious inventor has very pro perly secured the exclusive sale of it by patent. The description which Dr Wollaston has given of the Camera lucida is so short and perspicuous, that we shall make no apology for giving it in his own words.—We shall then conclude the article with an account of an instrument founded on the same principle.
" While I look directly down at a sheet of paper on my table, if I hold between my eye and the paper a piece of plain glass, inclined from me downwards at an angle of 45°, I see by reflection the view that is before me, in the same direction that I see my paper through the glass. I might then take a sketch of it ; but the position of the object would be reversed.
To obtain a direct view, it is necessary to have two reflections. The transparent glass must for this purpose be inclined to the perpendicular line of sight only the half of•45°, that it may reflect the view a second time from a piece of looking glass placed beneath it, and in clined upwards at an equal angle. The objects now ap peal' as if seen through the paper in the same place as before ; but they are direct instead of being inverted, and they may be discerned in this manner sufficiently well for determining the principal positions.
The pencil, however, and any object which it is to trace, cannot both be seen distinctly in the same state of the eye, on account of the difference of their distances, and the efforts of successive adaptation of the eye to one or to the other, would become painful if frequently re peated. In order to remedy this inconvenience, the pa per and pencil may be viewed through a convex lens of such a focus, as to require no more effort than is neces for seeing the distant objects distinctly. These will
then appear to correspond with the paper in distance as well as direction, and may be drawn with facility, and with any desired degree of precision.
This arrangement of glasses will probably be best un derstood from inspection of Plate CI X. Fig. 4. a b is the transparent glass ; b c the lower reflector ; b d a convex lens (of twelve inches focus) ; e the position of the eye ; and fghc the course of the rays.
In some cases a different construction will be prefera ble. Those eyes, which without assistance are adapted to seeing 'tear objects alone, will not admit the use of a convex glass ; but will, on the contrary, require one that is concave to be placed in front, to render the distant ob jects distinct. The frame for a glass of this construction is represented at i k, Fig. 6. turning upon the same hinge at h with a convex glass in the frame 1 m, and moving in such a manner, that either of the glasses may he turned alone into its place, as may be necessary to suit an eye that is long or short sighted. Those persons, however, whose sight is nearly perfect, may at pleasure use either of the glasses.
The instrument represented in that figure differs moreover in other respects from the foregoing, which I have chosen to describe first, because the action of the reflectors there employed would be more generally un derstood. But those who arc conversant with the science of optics, will perceive the advantage that may be derived in this instance from prismatic reflection ; for when a ray of light has entered a solid piece of glass, and falls from within upon any surface, at an inclination of only twenty ttt or twenty-three degrees, as above supposed, thc ft active power of the glass is such as to suffer none of that light to pass out, and the surface becomes in this casc.the most brilliant reflector that can be employed.