for Professor Wheatstone to supply the omission. The discovery of the projection of two obviously dissimilar pictures on the two retina when a single object is viewed, while the optic axes converge,1 led Professor Wheatstone to contrive the instrument to which he gave the name of Stereoscope.

It would be impossible in one evening to describe all the experiments which Professor Wheatstone devised to establish the facts which I have related, and the various curious and interesting phenomena which he observed during his investigations; for this information I must refer my audience to his memoirs on the Physiology of Vision. I must now proceed to describe the instrument which he found to be most convenient for enabling him to pursue his investigations in this branch of Optical Science.

DESCRIPTION OF THE STEREOSCOPE.

The first form of the Instrument consisted of two plane mirrors about 4in. square, inserted in frames and so adjusted that their backs form an angle of 90°. These mirrors are fixed by their edges on a board; two upright ends were placed on this board at equal distances from each opposite mirror; on the upright ends were fixed panels to which pictures were attached so that their corresponding horizontal lines should be on the same level. To use the instrument the observer must place his eyes as near as possible to the mirrors; the pictures must be adjusted so that the reflected images of the two pictures coincide and form one. For the purposes of illustration, the Professor employed outline figures only, for had either shading or colouring been introduced, the effects might have been supposed due to these circumstances. When the discovery of the Daguerreotype and the Talbotype was made known, Professor Wheatstone employed Photography to produce these illusions of solid objects in a much more efficient 1 Sir David Brewster states that this fact was known to Aquilonius, and to Baptista Porta.

The

manner than those previously drawn by hand. Stereoscope remained in this form for about eleven years. During this period very little was heard of it, except from its occasional exhibition to illustrate the theory of Binocular Vision by lecturers on Optics. In the year 1849, the subject was again brought before the public at the meeting of the British Association at Birmingham, by Sir David Brewster; he exhibited before the meeting a modification of the Stereoscope, to which he gave the name of Lenticular Stereoscope. Although I had in my possession for several years, one of the reflecting Stereoscopes with the outline geometrical figures, with which they were generally furnished, I well remember my astonishment at seeing a Stereoscopic calotype picture of some statuary, which Sir David Brewster exhibited after the business of the section was over. The convenient form and portability of this new construction, together with the advancing art of Photography, rapidly brought this instrument into favour.

We now come to the description of the Lenticular Stereoscope. The principal parts of the Lenticular Stereoscope, with the form of which you are all familiar, consist of two half lenses of 6in. focal length. These are cut out of one lens to ensure equality in foci. The annexed diagram, Fig. 1, will show how this is effected.

FIG. 1.

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The positions which these half lenses occupy in the instrument is shown in section at Fig. 2.

FIG. 2.

These semi-lenses are fixed on a frame, or box, in tubes, admitting of a little adjustment both vertically and laterally, and placed about 2 inches apart, (the average distance between the pupils of the eyes). Sir David Brewster, in describing the effects produced by this instrument, says: "When we view two dissimilar views or drawings of a solid object, through two such lenses, we are actually looking through two prisms, which produce a second image of each drawing, and when the second images unite, we then see the solid object they represent, and in order that these two images may thus unite without any effort on the part of the eye, the distance of the similar parts of the two drawings should be twice the separation produced by these lenticular prisms. There are several modifications of this instrument, some of which are placed before you; one by Duboseq., of Paris, in which prisms are placed above the two lenses, so that larger pictures may be viewed; and one called the Cosmoramic Stereoscope, by Knight, of London, in which lenses of larger diameter are employed; this form is very convenient and requires but little adjustment."

A very portable Stereoscope for the pocket is mounted in a frame, like a double eye-glass, and shuts up into a very small space. Some are made to fold up in leather cases, along with the Stereoscopic picture. The cheapest one is made by simply fixing the semi-lenses in a piece of wood. [Mr. Dancer exhibited specimens of these portable Stereoscopes, which were examined with interest by the members at the conclusion of his paper.] The original reflecting Stereoscope of Professor Wheatstone is perhaps the best form of instrument for pictures of above 6 inches square, and the portable form in which it is now made is bringing it into use for large Photographic views.

I must now proceed to note the method of taking Binocular Photographs in the Camera for Stereoscopic pictures. One of the best kinds of frames for the prepared plates is that generally used by the French operators. This frame will hold a prepared glass plate of the ordinary size of Stereoscopic pictures, 6in. by 44in. It has a sliding motion to the extent of 21 inches, and when the frame is drawn to one end, a portion of the plate can be exposed, on which one picture can be taken whilst the other end is protected.

I will imagine the Photographer has selected the subject of which he wishes to take a picture, and that he has at hand the requisite apparatus for the purpose. You are aware that two pictures are required to produce the effect of relievo in the Stereoscope; it is also necessary that these two pictures should be taken some distance apart. Several contrivances have been suggested to effect this movement; one of the most efficient is that of Mr. Latimer Clark; in principle it is the parallel ruler. This frame with parallel bars is fixed on a tripod stand, and the camera is placed on the two projecting pins. The camera is now to be adjusted to the view, and the frame with its prepared plate placed in the camera; the frame and camera must be drawn to the right hand side, and the cap of the lens being removed, the first picture is taken; the cap is then replaced and both camera and frame moved to the left hand, then the cap again removed and the second picture taken. If the same order in taking the right and left hand, which I have just described, is not attended to, the photograph when placed in the Stereoscope will have a very singular appearance; the most remote parts of the picture will appear in front of those in the foreground, in fact the right eye will view the left eye picture. A mistake of this kind can be easily remedied by cutting the plate in two and placing the proper picture before each eye.

There has been considerable difference of opinion as regards the proper distance through which the camera should be moved before taking the second picture. Sir David Brewster has published a table of degrees, through which the camera should be moved, decreasing according to the distance of the object to be photographed; for instance, at 5 feet distance from the object, move the camera 23° 32', at 20 feet only 5o 58'. Professor Wheatstone advised the pictures to be taken at 2 feet apart for every 50 feet distance of object. In following these directions the camera would require to be moved a distance of 50 feet after taking the first picture, supposing the object to be a quarter of a mile distant. Some three years since, this subject occupied my attention for some time. After having taken Stereoscopic photographs according to the directions published by those who had written on the subject, I arrived at this conclusion-if you desire the photographic pictures to present a truthful appearance when viewed in the Stereoscope, the camera must be moved through a space of 2 inches only, before taking the second picture, in fact, the average distance between the pupils of the human eyes. No doubt extraordinary effects are produced by increasing the angle, but the limbs and various portions of the figure exhibit a distorted appearance.

It is argued that some exaggeration is requisite to produce a striking effect, but when the eye has been accustomed to view Stereoscopic pictures taken at what I imagine to be the proper and natural angle, it becomes an easy matter to detect exaggerated effects. Some difficulty is experienced in taking satisfactory Stereoscopic pictures from life, when a single camera is employed. It is almost impossible for the sitter to preserve the same expression during the double operation; the movement of the camera is of itself almost certain to cause some disturbance. In taking a group, the chances of taking perfect pic