Carl Pulfrich

“ If every one had the same inter- pupillary distance there would be nothing more perfect than this stereoscope.
If in fig. 6 the two pictures L and R are interchanged in both pictures (a or b in fig. 11) , then the image-points for H are closer together than those for V; thus in stereoscopic vision H appears in front of P, and V behind it. No change is made to the relief by turning the picture upside down (c and d in fig. 11) . In fig. lid, the pictures are in the same positions as when the photographs are taken (Fi, F 2 in fig. 6) . Obviously transparent pictures can be easily reversed ”

“ This latter theory, however, was contradicted by H. W. Dove, who showed that a stereoscopic viewing was also possible with momentary i illumination of the object; and still less does it agree with the fact, to which Wheatstone first called attention, that facsimiles also have a stereoscopic influence, in spite of the fact that the images retain their position on the retina unchanged. Numerous experiments show the same result, and it follows that even a change of the angle of convergence is not always observed as a change of depth. ”

“ The power of perception of depth in man is most accurate. This has been ascertained by the approximately equal keenness of vision of all normal-sighted people and by the interpupillary distance. The angle which serves as a measure for the keen- ness of vision is that under which appear two neighbouring points of the object-space which are still seen by the single eye as a double point ”

“ At this distance objects are only still distinguishable from those lying behind them, which together form a surface but cannot always be seen as a surface because our representations of the depths of distant objects are not conclusively controlled by stereoscopic sight. ”

“ We will now investigate the conditions necessary for the perception of depth.
If the head is held still only one portion of space can be observed stereoscopically. The single eye, when moved, sur- veys, including indirect vision, a field which measures 180° in a horizontal direction, and 135 in a vertical direction. The two fields overlap and a smaller conical space is formed, with the nose as vertex (B V S in fig. 2) , in which both eyes can see simultaneously; and outside this space stereoscopic vision is impossible. The shape and size of this space are very differ- ent in men and animals. ”

“ Fig. 14.
and the exactitude of measurement needed. For measuring the pictures a binocular microscope, adjusted to the dimensions and the distance between the two plates, is used, and a fixed mark is placed in each image plane which combine in binocular view to a virtual mark in the three-dimensional image. ”

“ It is unimportant for perception where the filament mentioned above is illuminated. In order to see two objects lying close to one another it is not essential that the two image-points should be separated from one another by the distance of the two nerve fila- ments of the eyes. This happens whenever the line separating two objects passes through the two points (see fig. 4) . It is natural that the perception of depth has no fixed limits, for the position of the images shown in fig. 4 changes with the movement ol the eyeball, and the closer the two points are to one another, the more
Fig. ”

“ Fig. 8 large pictures, which can be purchased in book form. Fig. 8 shows diagram- matically the arrangement by which one picture is seen direct and the other in a mirror (H. W. Dove, Sir David Brewster and W. Rollmann) . The disadvantage attached to this, that the picture observed in the mirror must be reversed, can according to Pulfrich [1] be obviated by rotating the correct picture through 180 in its own plane and placing it in the position of the picture L and by using a so-called roof-prism in the place of the mirror.
Incorrect stereoscopic effects easily arise when using pictures. ”

“ The works of I. Harmer (1881) and F. Stolze (1884 and 1892) are of importance for the history of the development of stereo- scopic measurement. Harmer used a scale of depth consisting of a series of squares arranged one behind the other in order to measure in the stereoscope a picture of the clouds taken with a large base-line (about 15 metres) . Stolze placed gratings in front of the two semi-pictures of a mirror stereoscope, one of
Fig. ”

“ Fig. 13 shows how the lateral movement of the mark m^ is trans- formed in a movement towards and away from the observer in the three-dimensional image M. Fig. 14 shows the theory of measuring a stereophotograph. The axes are horizontal when the photograph is taken, and the plates are in one plane. ”

“ For a large number of experi- ments of this sort — mountain photography (Von Hubl, &c.) , coastal measurements, photographing a battle from a ship, geodesy, study of the waves (Kohlschutter, Laas) , the trajectory of a shot (Neuffer, Krupp, Neesen) , the use in building railways or on voyages of discovery, &c. — the stereocomparator has
new fields are being constantly opened up for it. ”