( 834 ) 
I. Polyphany. 
The images we have to consider are central projections. The 
connection between the two stereoscopic half-images is expressed by 
the fact that the exposing-distance is the same for both, the centres. 
of the two projections being separated by a definite distance which 
we call the base length. It will be readily understood that we may 
make any number of half-images, from any number of points which 
satisfy these requirements. Any given half-image is not limited to 
only one corresponding half-image, but the number is unlimited. 
The question thus arises, whether it may not be of advantage in 
practice, to make a stereoscopic exposure from more than two points. 
A simple experiment will answer this question in the affirmative. 
If we hold a cylindrical stick in a horizontal position at a short 
distance before the eyes, both eyes receive one and the same image 
of the stick and that what is hidden behind the stick from one eye, 
is also hidden from the other eye. 
It is quite different however, if we hold the stick in a vertical 
position, for then the left eye sees behind the stick what is hidden 
from the right eye, and the right eye sees what is hidden from the left. 
A similar phenomenon may be observed with the Réntgen rays. If 
in taking a skiagram of any object we place a thick wire parallel 
to the base, there will be no representation on either plate of what 
is situated before or behind the wire. Hence it is impossible to decide 
whether this wire is placed before or behind the object. If however 
we place the wire perpendicular to the former direction, one is 
able to see with perfect clearness, at what depth the wire is situated. 
Figure I shows a skeleton hand, in which the fingers are placed 
in a very unnatural position. Four exposures have been made, the 
respective projection-centres making a square of 65 millimeters in a 
plane parallel to the plate, Under the hand in a transverse direction 
is a metal staff. Holding the plate so that the fingers point upwards 
and examining the two lower images through the stereoscope, one 
cannot distinguish whether the metal staff is behind or in front of 
the hand. 
Likewise if we look at the two uppermost pictures, one is also 
in doubt as to the position of the metal staff. Mathematically speaking 
we has obtained all the information derivable from these four pictures. 
Psychically speaking however this is not the case. When we turn 
the plate on its side and examine the upper pair of pictures by 
the stereoscope, we obtain at once an accurate impression of the 
position of the metal staff. We may in the same way examine the 
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