In taking stereoscopic pictures, the axis of the camera in its different positions should strictly preserve its parallelism, however wide apart the stations may be ; otherwise, there will be more or less difficulty in properly combining- the images seen in the stereo scope. This vvill be evident from the following considerations :— In Fig. 7, let A B be a horizontal line above the level of the eye, (the parapet of a high building, for in stance,) and let L R be stations from which stereoscopic views are taken. Let the axis of the camera be directed, in both cases, towards the same point C. Then, the line A B in the pictures will not be hori zontal, but will tend to a vanishing point. In the picture taken from L, (the left station,) the vanishing point will be on the right-hand side beyond B, and the line A B will be inclined, as shown in the figure beneath L. In the picture taken from R, (the right station,) the line A B vvill incline towards the left side, and tend to a vanishing point beyond A, as shown in the figure beneath R. When, therefore, the two pictures are mounted and placed in the stereoscope, the points A, A, will not be on the same horizontal line, and it vvill be found impossible to combine either the two A's or the two B's, so as to produce stereoscopic effect, without turning the head, so as to bring, from the points A, A, the right eye downwards and the left eye up, and vice versd, for the points B, B. In other words, it will be impossible to com bine either the two A's or the two B's, with the eyes on their normal horizontal line.
For, suppose we call R the right eye and L the left, and consider the line L R which joins the eyes to be horizontal ; then, if we join the optic axes L A, R A, and produce them, it is evident that they cannot possibly intersect in a common point, unless the two points A A are in the same horizontal line ; for, otherwise, the lines R A, L A, will not lie in a plane, and straight lines which do not lie in the same plane, cannot possibly meet in a point.
The convergency of the two directions of the axis of a camera, wheri taking duplicate views for the stereoscope, is therefore shown to be wrong in principle ; and if the difficulty in uniting the images is not perceived in practice, it may perhaps be that the error is in most cases trifling in amount, and the head, by a series of small motions, brings the eyes into a succession of proper positions for uniting the several points of the images.
The scientifically correct form of stereoscopic camera is that in which the axis of the instrument always preserves its parallelism, so that the two pictures are taken on th,e eagle plane. When the pictures are small, and the stations near together, this may be very conveniently effected by combining two cameras in one, and taking both pictures on the same plate ; and a great merit of this arrange ment is, that both pictures can be taken at once. In fact, so great
an advantage is this, that no objects which move rapidly can possibly be taken in any other way. The double instrument, called the " double-lens stereoscopic camera," (or erroneously, by some persons, the " binocular camera,") is therefore, in every respect, the most scientific instrument to employ for obtaining stereoscopic pictures.
A good form of the instrument, for taldng the small pictures for the lenticular or cosmorama stereoscope, is shown in Figs. 8 and 9.
Fig. 8, scarcely requires explanation. When a stop is used, it should be put immediately in front of the first lens. Without a stop, instantaneous pictures could be taken in a good light. The tubes which carry the lenses can be moved backwards and forwards in focussing, by means of a rack and pinion in the sides of the camera. Either tube may be moved independently of the other. The front of the camera has two circular openings to admit the pencils which produce the images, These are covered and un covered by the revolution of a circular plate, (see Fig. 9,) which has two corresponding openings, and turns about an axis in the centre of the front of the camera. At the bottom of this revolving plate is a heavy handle, which, by its weight, keeps the openings in the plate in a vertical position, and the holes in the camera covered. By a rapid semi-revolution of this plate, a nearly instantaneous exposure may be g,iven to the pictures.
It only remains to add a few words with respect to the principal dimensions of the camera.
The equivalent focal length of the lenses should be five inches, and they should be five inches from centre to centre. This distance between the stations is a mean which will suit a very large class of subjects, including portraits and views, in which there are near as well as distant objects. The pictures are circular, four inches in diameter, and five inches from centre to centre. The positive prints, when cut and trimmed, are reduced to the usual size for the ordinary stereoscope, or, when exhibited as circular pictures, are suitable for the large cosmoramic stereoscope, the lenses of which are quarter lenses cut from a large one five inches in diameter. The glass plates suitable for this camera should be ten inches long by five inches wide. The slide should be a non-reversing slide, in which either the coated side, or the back of the plate, may be presented to the lens.