The use of two separate shutters is avoided for the same reason, apart from the fact that they will be more expensive than a stereoscopic shutter with two openings.' 839. Several makers sell stereoscopic cameras which can be used for taking a single image, thus obtaining an elongated view com parable to some extent with a panorama. In these cameras, generally called stereo-panoramic cameras, the centre partition is moved against the body when one lens is moved opposite the centre of the plate, either by sliding the lens board horizontally or by rotating an eccentric platform carrying one of the lenses. The normal lenses used for each image scarcely cover the considerable angle of field thus used unless greatly stopped down ; the image obtained at the edges of the field is of poor definition and luminous intensity. For example, the lens of 3 in. focal length covers, on one of the images of a 6 x 13 cm. stereogram, an angle of 56, whereas it must be capable of covering an angle of 93' to use the full size of the plate.
It is well known by users of such cameras that circumstances justifying the use of this panoramic arrangement are extremely rare.
840. Stereoscopic Cameras Avoiding the Need for Transposing the Negatives. The production of stereoscopic images capable of being viewed directly in an ordinary stereoscope by reversal of the original images is a problem whose solu tion has been sought from the beginning of stereoscopic photography on Daguerreotype plates. It was solved by A. Claudet (1853) by fitting each of the lenses with a mirror or prism for optical reversal of each image.
Many attempts have been made since then to bring this type of camera into every-day use, notably those of J. Carpentier (1895), by means of right-angled prisms placed between the front and back components of the lens ; by j. A. Tournier (1902), by means of reflectors inside the camera, projecting the images on to a roll film wound round the partition separating the two dark compartments; and by A. Daubresse (1902), by means of two reflectors inside the camera, throwing the images on a sensitive film placed between the lenses.
In spite of the advantage of such a camera in stereoscopic colour photography on mosaic colour plates, there being no need to cut the negatives, it has never been popular.
841. Cameras Fitted with Mirrors for Register ing Simultaneously the Two Stereoscopic Images, Using Only One Lens. In a camera taking pictures at least 3i X 41 in., by a suitable arrangement of mirrors in front of the lens, two separate images may be formed of the view as seen from the position of the images of the two half-apertures of the lens in the two mirrors.
Such a duplicating arrangement, consisting of two mirrors making a very obtuse angle with one another and having their joining line vertical and situated on the axis of the lens, was devised in 1853 by F. A. P. Barnard for stereoscopic
photographs on Daguerreotype plates. Each of the two images of the pair is turned round by a single reflection of the incident rays. If used to obtain negatives to be finally printed by contact, this arrangement is open to the objection that the images are reversed as regards right and left.
A duplicating system, using two pairs of mirrors, recalling the stereoscope of Cazes (§ 829), was made in 1894 by T. Brown. Each of the im ages being reflected twice, the negative obtained is similar to an ordinary stereoscopic negative.
The mirrors of these duplicating systems are very delicate, since they require to be silvered on the surface to avoid the double reflections obtained with an ordinary mirror silvered on the back. Moreover, these mirrors reflect at most 90 per cent of the incident light (or 8o per cent after two reflections). Also the lens forms each image with only half its surface, thus reducing its " speed " to one-half its normal value.
These arrangements are not at all suitable if one wishes to conform to the best geometric conditions ; the two half-pencils which are incident on the lens only have common regions if the two axes (virtual) are convergent.
842. Stands for Two Exposures in Rapid Succession. For stereoscopic photography of inanimate objects by means of two successive exposures with any camera, a number of arrangements have been put forward which may be fixed to a tripod and automatically ensure parallelism between the optical axes in the two positions and the desired separation between the two view-points.
The following arrangements, among others, have been used : A board with stops indicating the two positions of the camera; a board with a carriage running on guides between fixed or adjustable stops, the camera being fixed to the carriage ; a board to which is secured by means of four movable rods another board, the rods and boards forming two parallelograms ; this ensures parallelism in the extreme positions of the movable board.
843. Stereoscopic Photography, Using a Large Base. There are two cases to be considered, the study of a distant object of small depth, but no objects situated at intermediate distances, and the photography to the best advantage of a group of objects situated at all distances from the base and partially obscuring each other from view. As typical examples of the two respective cases may be mentioned : (I) vertical stereo scopic photography, from an aeroplane flying at great altitude, of an expanse of country, or the plan of a works, and (2) the stereoscopic photo graphy of a landscape from ground stations.