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negatives need not necessarily be taken with the same angle of 
aperture. The skiagram may be taken of only a small area, while 
the photographic picture may comprise the whole surroundings. We 
are thus enabled to use small Röntgen plates, a matter of some 
importance from an economical point of view. 
Symphany may also be carried out by using skiagrams that have 
been reduced in size, the photographic negatives being reduced in 
the same proportion. By using a “Verant lens-stereoscope” these 
reductions may be reconstructed of the original size. This is done 
by placing one pair of pictures behind the lens, and the other pair 
on the upper part of the stereoscope (see Figure 9). The symphany 
is effected by means of a transparent mirror placed at an angle of 45°. 
It is evident that we may combine symphany with polyphany. 
We may now proceed to describe the methods suitable for radios- 
copie examination with the fluorescent screen. 
I. Metaphany. 
It is often too much trouble for the Röntgenologist to make two 
stereoscopic pictures. Davipson’s method is but seldom employed in 
practice, probably because it is too laborious and requires a separate 
apparatus. According to this method the right and the left half-images 
appear alternately on the screen in rapid succession, the eyes being 
alternately eclipsed in synchrony, so that each eye sees only the 
corresponding half-image. This appears to be the only method suitable 
for stereoscopic vision on the screen, if we adhere strictly to the notion 
that the stereoscopic sensation consists of two slightly different central 
projections. We have already shown that this definition is too narrow, 
since the impression of relief is enhanced if we take more than 2 
centres of projection. This is also the case in ordinary vision, where 
the sensation of relief is produced by the movements of the head. 
It is not however, the only way in which an impression of relief is 
brought about psychically, for a one-eyed person is also able to gain 
the sensation of relief by moving his head to and fro, so that the 
object may be viewed from more than one side. These different 
impressions are translated psychically into the relief-image. This 
resembles to the monocular stereoscopy described by Srraus. It may 
also be observed in the kinematograph, where an image, such as a 
ship, is not shown up in good relief, until it is seen turning round 
on the screen. 
In figure 10 I have depicted the formation of the stereoscopic 
image on the screen. In Davipson’s method the eyes must be exactly 
