There are also the simplified stereoscopes used for paper prints, automatic stereoscopes with metal frames for the pictures mounted on end less chains (of which some models do not function until a coin is placed in the slot), stereoscopes for films run from one spool to another, and the cabinet stereoscopes with interchangeable maga zines (made about 190o) in which one picture is substituted for another by pressing a lever.
828. Complementary Stereoscope. The essen tial condition for the reconstructed object being identical with the objects itself is that each of the images is seen at a distance equal to the principal distance, or at least by means of an eyepiece of focal distance 1 equal to this principal distance (§ 25, footnote). When one restricts oneself to the photography of distant objects, the principal distance is practically equal to the focal length of the objectives used to take the photographs, and the above condition is then satisfied if the eyepieces of the stereoscope have the same focal length as the objectives of the camera ; it is said then that the stereoscope is complementary to the camera used. I The general practice in stereoscopy is to fit the camera with lenses of very short focal length, and it is sometimes difficult to Iit the stereoscope with such short-focus eyepieces, owing to their higher cost. The greater curvature of such lenses compels the use of a smaller number at once ; the field is limited owing to the con siderable thickness of the edges, and also to the distortion of the marginal regions. Thus the eyepieces of stereoscopes are frequently of greater focal length than the camera lenses, resulting in exaggeration of the foreground and deformation of the reconstructed object, and tending to give the effect of theatre scenery in successive planes.
When the difference of focal length is not con siderable one can often add an auxiliary lens (§ 118) to each eyepiece of the stereoscope, the power necessary being the difference of powers (§ 6o and 70) of the camera lens and stereoscope lens.' Meniscus lenses of diameter equal to the stereoscope lenses are chosen for preference, and are introduced into the mounts. They are kept out of contact with the stereoscope eye pieces by means of cardboard rings of sufficient thickness, and placed next to the eyes with their convex faces outwards (C. Schitz, 1914).
829. Stereoscopes Using Symmetrically-placed Mirrors. The stereoscope of C. Wheatstone (1838) employed only two mirrors, Al and M' (Fig. 210) at right angles, and placed at 45" to the direction of vision. The two photographs constituting the pair are represented by AB, A'B', parallel to the plane bisecting the two mirrors. Under these conditions the eyes C) and 0' see at and A the reversed images of AB and Ale , two homologous points is and n' being seen respectively at and and giving thus the illusion of a point of the recon structed object at the intersection of the rays On, and The necessity of using reversed images has somewhat reduced its popularity, in spite of the fact that there are no limits to the size of the photographs which may he viewed.
Inspired by the telestereoscope of Ffelmholtz (§ 8i5), 1.. Cazes (1895) made a stereoscope using two pairs of mirrors, inneMM' (Fig. 211), for viewing stereograms of large size having the pictures placed side by side in the same plane. The " virtual " eyes are brought each time opposite the principal Points Pi" (their separa tion may vary from 5 in. to 20 in.) by sliding the mirrors Mir on rails RST (graduations on the rails enable one to make sure the mirrors are symmetrically placed). The whole instru ment is carried on an extensible vertical column to ensure equality between the distances 0,P, 0,'P', and the principal distances of the per spectives (up to 25 in.). Converging or diverging lenses may be placed in front of the eye mirrors, to regulate the apparent distance of the fore ground,' according to a table on the instrument. The effort of accommodation imposed on the eves is then equal to the effort of convergence when viewing the object direct. 2 F. Drouin (189b) has proposed the use of prisms of quadrilateral section (totally reflecting the light from two opposite faces), to be held in front of one eye for examining normal stereo scopic pairs or pseudoscopic pairs (not trans posed). Such instruments have been used at various times for viewing stereoscopic projec [ions.