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opposite the anticathodes and the same distance in front of the 
screen, as the anticathodes are behind it. In this way we may 
obtain a complete mathematical reconstruction with this difference, 
that the stereo-image is a mirror-image of the original object. 
A little consideration will show that using one eye only when this 
eye moves from L’ to R’ to and fro, while the anticathode R 
follows the movement from L to R, the eye receives the same 
impression, as if there were a relief-image on this side of the screen. 
Psychically one receives a correct impression as to the position of 
objects, which lie in front and which behind. The distance between 
R and J, which we supposed to be 65 millimeters may be varied. 
the displacement may be increased in any direction, transversely, or 
vertically, backwards or forwards, so long as we are careful to keep 
the eye exactly opposite the anticathode and the distance of them 
from the screen identical. When one has realized the fact that we 
are not looking at the object itself, but at the mirror-image on the 
screen the appearance of depth and relief is really surprising. 
The simultaneous movement of the eye-piece and the anticathode 
would require a rather complicated apparatus, but the matter may 
however be simplified, if we restrict the forward and backward 
motion and permit the anticathode to move only parallel to the 
screen. The ordinary orthodiagraph may be adapted for this purpose. 
In the orthodiagraph the focus tube always moves parallel to the 
screen. The eye-piece may be connected to the focus tube in such 
a manner that its aperture is always opposite the anticathode, both 
focus-tube and eye-piece moving together. 
The apparatus may be still further simplified by restricting the 
downward movement and simply hanging the focus tube by two 
strings, which only permit a pendulum movement. By this device one 
may obtain a good impression of the depth and of the relief, when 
the head follows this pendulum movement of the focus-iube and 
eye-piece. In figure 11, S is the screen, A,, A, and A, are the anti- 
cathodes and O,, O, and O, the eye-piece in various positions. It is 
evident that whatever the position of the anticathode the point B 
which is in contact with the screen, always retains its position. The 
point D however, which lies nearer, is projected in turn at d,, d, 
and d, and the eye, following the movement, receives the impression 
that whereas B is situated on the screen, the point D lies at a 
different depth viz. at D’. 
This method serves not only for making a psychical relief-impres- 
sion, but also for the stereogrammetrical determination of distance. 
If we suppose the eye to be at O, and we direct the line of vision 
