THE STEREOSCOPE The fundamental property of stereoscopic vision, or simul taneous vision with both eyes, is the direct perception of relative distances of near objects. Even by the use of one eye it is pos sible to obtain an impression of depth or relative distance, but this is the result of experiences and other considerations. When using both eyes each eye is fixed and accommodated to one particular spot of the object. The angle made by the two lines connecting the right and left eye respectively with the spot chosen on the object varies with the distance of the object. For an object at the distance of io in.
(normal vision) the angle would be about 15 °, and it decreases as the object moves away from the observer, until it reaches zero when the object is at an infinite distance. The two images of a near object formed on the retina are dissimilar as is quite evident, and, as each eye transmits its respective picture to the brain, their dissimilarity creates the perception of depth or dis tance.
The greater the distance of the object the less will be the dis similarity between the two images. If we take 1 minute as the usual angle which two lines must subtend at the eye to be seen separately, rather than as one line formed by a blending together, it has been shown that stereoscopic vision for a normal sighted person ceases between 40o and 55o yd., according to the interpupillary dis tance. It is possible to convey the im pression of depth by offering to both eyes plane representations of three dimensional objects, by, for instance, taking two photo graphs from the points of rotation of the eyes and by so arranging them in an instru ment that each eye sees its respective pic ture without being able to see the other one. Such an instrument is called a stereo scope.
Construction of Various Types. C. Wheatstone first observed the stereo scopic effect, in 1833, and was the first scientist to construct such an instrument, which he described in his paper in 1838. A prin ciple of the instrument is shown in fig. 2o. A later type of his stereoscope, evolved in 1852, differs from the original model in that the pictures can be placed at different inclinations to the mirrors and at different distances from them, in order that the pictures may be ob served under exactly the same inclination of the image and the same angle of con vergence as when the picture was taken. Other mirror stereoscopes were made by H. W. Dove, Sir David Brewster, and others. These mirror stereoscopes had no practical result worth mentioning on ac count of their awkward shape and the dif ficulty in obtaining equal illumination of both pictures. It was also inconvenient that the pictures had to be placed separate ly and reversed in the apparatus. The dis advantage that the picture to be observed in the mirror must be reversed can be ob viated by rotating the correct picture through i8o° in its own plane and placing it in the position of the picture L (fig. 21) and by us ing a so-called "roof-prism" in the place of the mirror as sug gested by Pulfrich. Sir David Brewster took up the stereoscope in 1849, and in a paper read in that year proposed his prism lens stereoscope in which he used eccentric portions of double convex lenses for viewing the half pictures. The first instrument of this type was made by A. Ross in the same year, but created no interest whatever amongst the public. It was only after J. Dubosq of Paris had taken up the regular manufacture that the instrument became very largely used. The causes of its success were its convenient form and the fact that a series of adjusted stereoscopic pictures (landscapes, machines, etc.) could be observed in rapid succession.
Brewster's stereoscope made an observation of stereoscopic pictures possible when the distance between identical points on both pictures was considerably greater than that between the observer's eyes. If the lenses shown in fig. 22 on the focal plane of which the image is formed are large enough, and the distance between the image points and is not greater than the distance between the centres of the two lenses (avoiding the divergence of the axes of the eyes), then the distance between the eyes is secondary and the observer sees the distant points with the axes of the eyes parallel. These apparent advantages how ever are counterbalanced by the fact that the picture seen through the lenses is eccentric and consequently an incor rect impression is obtained and an aber ration in the three dimensional images occurs. Wheatstone showed later in FIG. 20.-WHEATSTONE'S his controversy with Brewster that this STEREOSCOPE disadvantage in the lens stereoscope could be avoided by ad justing the lenses and distant points to the distance between the observer's eyes. This same condition was fulfilled in the "double verant," constructed by V. Rohr and Koehler (1905), in which the lenses in accordance with A. Gullstrand's rule are so arranged that the centre of rotation of the eye always coincides with the nodal point of the lens. If everyone had the same inter pupillary distance there would be nothing more perfect than this stereoscope.
Helmholtz showed in 1866 that stereoscopes should be so de signed that the image of, for instance, a landscape should be shown in the stereoscope to appear to be at infinite distance; he designed his lens stereoscope accordingly. The instrument is shown in fig. 23, and consists of a box similar to Brewster's stereoscope, but it contains complete convex lenses for the eye pieces, the upper lens of about 5 in. focus, the lower of 7 in. focus. Combined these lenses gave a focal length of 21 in. to the eyepieces. The complete combination was used for viewing pictures on glass which had been photographed with a pair of lenses having a focal length of 22 inches. Two adjustments were provided for the eyepieces—one in the direction of the optical axis for focusing, and another at right-angles thereto for the interpupillary distance. If the stereoscope was to be used for viewing pictures which had been taken with 5 in. focus lenses, the upper lenses of the eyepieces alone were used, after the lower element in each eyepiece had been removed.
A reversed stereoscopic effect can be obtained by the use of Wheatstone's pseudoscope (fig. 24). If two right angle prisms are placed in front of the eyes with their hypothenuse surfaces parallel, or nearly so, and in line with the direction of sight, and the apparatus be directed on an object in the middle distance, then objects farther away will appear small and near, while objects near will appear larger and farther away. These pseudo-stereo scopic phenomena are of great importance for the study of the principles of stereoscopy, for they demonstrate that the per ception of depth can be aided by direct presentation and hindered by reverse presentation.
The problem of making one stereoscopic picture visible to several people simultaneously can be met in various ways, most simply by portraying the two stereoscopic pictures in different colours one close to the other and giving each observer spectacles of different coloured glass or other transparent material with which it is only possible to see one picture with each eye. The latest development of this method has been the presentation of stereoscopic cinematograph films to large audiences. The film is printed in two colours and spectators are provided with suitably coloured spectacles.
To A. Rollett (1861) is due the merit of constructing the first stereoscopic measuring scale. It was a form of ladder, apparently extending into space, whose rungs gave the distances of the ob jects. J. Harmer (188i) used a scale of depth, consisting of a series of squares arranged one be hind the other, in order to measure in the stereoscope a pic ture of the clouds taken with a large base line of about 15 metres. N. F. Stolze (1884 and 1892) placed gratings in front of the two half pictures of a mirror stereoscope, one of which could be moved by a micrometer. He thus discovered the device called the "travelling mark." Of the practical application of stereoscopy we may note the stereoscopic rangefinder already dealt with under the paragraph for telescopes, the stereocomparator devised in 1901 by C. frich, and the stereoscopic measuring machine, invented by H. G. Fourcade of Capetown 1902, which is similar to the parator in many points. These instruments inaugurated the successful measurement of the distances of objects in space. ment is not made on the objects themselves but on photographic plates which are taken with special instruments, field and stand phototheodolites, at the extremities of a base-line which is always selected ing to the distance of the object and the exactitude of measurement needed. For measuring the pictures a binocular scope adjusted to the dimensions and the distance between the two plates is used, and a fixed mark is placed in each image plane ; these marks combine in binocular view to a virtual mark in the three dimensional images. If the plates are correctly adjusted the so-called travelling mark can be placed on any point of the landscape, and then used for the measurement of solidity of the objects or the production of plans and models, just as formerly, for example, the measuring staff was used for geodetic observations, with the difference that in the parator the mark is regulated by the observer only, and is not hindered in its movements by any undulations, etc., of the land.
Since 1904 binocular observation of stellar plates to determine differences in the images of the objects reproduced has been gradually discarded for the method, derived by Pulf rich, which consists in the monocular observation of the two plates in the stereocomparator with the assistance of the so-called "blink" microscope (fig. 25). In this microscope the two pictures are seen simultaneously or individually by alternately opening the screens and All differences of the images are immediately distinguished by a sudden oscillation of the image point, or by a sudden appearance and disappearance of single points, as in the case of flashlights at sea or the modern illuminated sky lights in towns, and there is now no difficulty in discovering new planets, comets and variable stars by this method.
C. Pulfrich (1912) describes a new form of reflecting stereo scope (fig. 26) in which the main object of the design was to pro vide stereoscopic apparatus for the examination of large pictures, notably those produced for stereophotogrammetric work and photographs taken by aircraft for the purposes of survey, etc. The instrument resembles the Wheatstone stereoscope (fig. 2o), but with the simple reflecting mirror replaced by two mirrors, set at an angle in front of the observer's eyes, vvhereby the rays are reflected twice and deflected through an angle of 12o°. The two mirrors in each half of the instrument have with advantage been replaced by a prism made from a solid block of glass. The instrument can be used for paper pictures or for plates, and is arranged with revolving plate holders for the orientation of photographs taken from aircraft.