(1) The camera lucida is an apparatus which renders great services to landscape painters by permitting them to see upon their canvass or drawing-paper the landscape that they wish to reproduce, and to sketch its outlines with an accuracy and rapidity that cannot be attained by means of the unaided eyesight. For reducing or enlarging drawings, maps, plans, etc, the camera lucida also gives excellent results. In short, this instrument forms part of the professional tools of the majority of artists, designers, engravers, etc.
The camera; lucidae invented by Wollaston have since been more or less improved upon, but all are based upon the same principle. They consist of a right-angled triangular prism, one of whose faces is covered with a small mirror. The rays, proceeding from the object whose image it is desired to see, first meet the prism, where they are refracted at their entrance and exit and then strike the mirror, and from this are reflected so that the draughtsman receives them in the direction of the sheet upon which he wishes to draw, and is thus enabled to trace their contours with a pencil. But a Wollaston camera lucida costs 1-5/. Now it is possible to obtain the same effects as are given by this apparatus, by using a simple mirror, or any bit of silvered glass, this fact being due to a physiological peculiarity of our vision.
When we look at an object, each of our eyes perceives its image, but the two images are superposed, and we thus have a perception of but a single object. If, by a slight pressure upon one of our eyes, we move the globe of the latter, while looking at the same object, the two images will be perceived separately, or, in other words, we shall see double.
It is probable that animals whose eyes have different directions, those for example that have eyes at the side, like many herbivora (hares, gazelles, &c), or that carry them upon peduncles (like crustaceans), do not perceive superposed images as we do.
It is due to such superposition of images that when we station ourselves before a sheet of white paper affixed to a wall, and turn so as to face it, it is possible, by looking with one eye into a small mirror, to see upon the paper, by means of the other eye, a reflection of the object situated behind us, and to thus easily follow or trace its outlines.
It is a very simple matter to get up a camera lucida upon this principle.
As for the arrangement of the apparatus, we may affix a small mirror with wire to the cover of an open sketch-book, and so place ourselves that we may, with the left eye regarding the mirror, see with the right a reflection of the object that we desire to draw. This image will be seen upon the vertical part of the drawing- paper in front of us, and we may then follow it in all its outlines and details, as we would do with an ordinary camera lucida. (La Nature).
(2) In all forms of camera lucida are more or less defects, such as limitation of field, distortion, indistinctness of image or of drawing-point, awkwardness of position, etc. Being engaged in endeavouring to simplify and perfect the construction and adjustment of Wenham's high-power binocular prism, it occurred to me that his arrangement of prisms might be modified, so as to be available as a camera lucida in which the defects of the forms hitherto made would be considerably reduced if not entirely eliminated.
Assuming a 45° inclination of the microscope to be the position most generally convenient for drawing, I drew on a large scale the system of prisms which appeared to be suitable for a camera lucida. Messrs. Ross undertook to construct the prisms to my drawings, and the apparatus was found upon trial to answer my expectations fully. I am induced to describe it because it has also met with much approbation from micro-scopists, who were previously disinclined to believe in the possibility of any new device at the present day, which should be substantially better than the numerous older forms which apparently exhausted the subject.
It is well, known that all forms of reflecting prisms acting by means of one reflection are extremely sensitive in regard to the position of the mirror in relation to the microscope, as also in a less degree in relation to the eye; the slightest deviation from the normal position in many cases entirely destroying the effectiveness of the apparatus. For this reason earners lucidae acting by one reflection have not found favour, though their apparent simplicity has induced the construction of many such forms.
In order to obviate the difficulties incident to the use of one reflection; many devices have been made acting by two reflections, and where these have been so contrived as to act like parallel mirrors, the reflected image has possessed the advantage peculiar to this principle, of being practically insensitive to slight differences of position relative to the microscope or to the eye, remaining in fact stationary within a considerable range of adjustment, as in Wollaston's camera lucida.
My device (Fig. 368) consists of a combination of a right-angled prism (Fig. 369), ABC, and a rhomboidal prism D E F G, so arranged that when adjusted very nearly in contact (i.e. separated by only a thin stratum of air) the faces B C and D E are parallel, and consequently between D E and B E they act together as a thick parallel plate of glass through which the drawing-paper is viewed. The rhomboidal prism is so constructed that when the face GF is applied at right angles to the optic axis of the microscope, the axial ray H passes without refraction to I on the internal face E F, whence it is totally reflected to J in the face D G. At J a part of the ray is reflected to the eye by ordinary reflection in the direction J K, and a part transmitted to J on the face A C of the right-angled prism. Of the latter a portion is also reflected to K by ordinary reflection at J'. The hypo-thenuse face A C is cut at such an angle that the reflection from J' coincides with that from J at the eye-point K, thus utilising the secondary reflection to strengthen the luminosity of the image.
The angle at G is arranged so that the extreme marginal ray H' from the field of the B eyepiece strikes upon D G at a point just beyond the angle of total reflection, the diffraction-bands at the limiting angle being faintly discernible at this edge of the field. This angle gives the greatest amount of light by ordinary reflection ihort of total reflection.
By this arrangement the Ramsden. circle over the eyepiece comes just above the camera lucida, and the field of view is not in any way reduced; all that can be seen directly through the B eyepiece (say 30° of field) is perfectly depicted in the camera lucida, whilst the drawing being viewed direct is of coarse not cut down in field.
In practice the microscope should be inclined about 45°, and the image accurately focused through the eyepiece as usual. The camera is then slid on the eyepiece and pushed down more or less until the microscopical image is seen distinctly and the illumination of the field is equal throughout. The drawing-paper is placed on the table immediately under the camera. The observer will then see the microscopical image projected on the paper, at the same time viewing the pencil-point directly. The whole pupil of the eye it available for both images, the diaphragm on the apparatus being considerably larger than the pupil. It may be necessary to balance the illumination either by subduing the light in the microscope or by increasing it on the drawing paper. It will generally be found that when the object is in a luminous field the light on the object (especially with lamplight) may be advantageously subdued by ground glass or similar menus. The eye may be removed as often as required from the camera, and the work reson.
menced without the slightest shifting of the image; and with properly balanced illumination, fully shaded drawings can be made with very little practice. The drawing-paper should in every case be placed at the distance of distinct vision, either using spectacles or not. If the vertical position of the microscope be preferred, the drawing-paper may be inclined 45", either in front or at the side of the instrument. For very accurate drawings in nil azimuths, the drawing-paper should of course wholly coincide with the plane of the optical image, as with every other form of camera lucida. A spring clip is provided in which a screen of black paper may be put to shade the eye not
This form of camera lucida can be modified so as to project the image at any desired angle. It can be need with the dissecting microscope or hand-magnifier, also on a stand for architectural or mechanical drawings. (H. Schroder.)