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object lay of the same size and in the same place, so that we 
may say that the stereo-image is congruent with the original 
object. Figure 5 is a representation of the course of the rays in 
the double mirror-stereoscope corresponding to the HeLmMHor7z tele- 
stereoscope. It is evident that the eyes at L and R are unable to see 
the object lying before the plate P, firstly because the small mirrors 
St, and St, are opaque and secondly because the photographic plates 
P, and P. obstruct the view. The first difficulty may be obviated by 
using semi-transparent mirrors of thin plate-glass. The second difficulty 
may be obviated by making the plates somewhat narrower, or 
placing them farther apart. By this means we are able to see at 
the same time the original object and the stereo-image on the same 
spot. In the human body one may see the bones through the skin 
in their exact relative position. This may serve as a guide to the 
surgeon during an operation, since he is able with mathematical 
accuracy to apply his scalpel to the actual spot. The course of the 
rays is represented in fig. 6. The great drawback for the surgeon 
from a practical point of view is that the two plates P, and P; impede 
the field of operation. This may be easily remedied however, for 
the mirror-stereoscope admits of all sorts of variations. For instance 
in fig. 7 the negatives are situated above the observer instead of 
below, and at such a distance apart as not to interfere with the 
surgeon’s head. This has another advantage in that the photographic 
plates lie horizontally above the surgeon and are therefore well lit 
up since the operating-theatre is as a rule lighted from above. 
It depends merely upon the degree of illumination, whether the 
Röntgen-image is more visible than the part to be operated upon, or 
whether the reverse is the case. The degree of illumination may 
easily be regulated, by diminishing the transparency of the small 
mirrors by a screen of smoked glass, enhancing the relative clearness 
of the Röntgen stereo-image. This method, i.e. the contemporaneous 
presence of the object itself together with its Röntgen-image may 
be termed Symphany. 
An important province of Röntgenology is stereogrammetry, the 
measurement of the depth of a foreign body. For this purpose I 
have designed an instrument which I may call the Symphanor, 
fig. 7. Let us suppose, that we cannot get at the object itself but 
that in its stead we have the virtual stereo-image. We may place a 
divided rule across any diameter of this virtual stereo-image, and 
thus measure the exact distance between any two points. This method 
I call Symphanometry, a method, which should prove of use for 
the record of scientific measurement as for instance in craniometry, 
