( 887 ) 
if we have a pair of stereoscopic skiagrams of a cranium we may 
measure the diameter of the stereo-image in various directions by 
means of the symphanator. We thus possess in the stereogram a 
means not only of reproducing the general psychical impression, but 
also a means of registering its mathematical properties. An orthogonal 
parallel-projection of this virtual image, which in Röntgenology would 
correspond to the orthodiagram, may be made by tracing taking care 
to hold the pencil vertically. Similarly a wax or clay image may 
be modelled by placing the plastic mass in just a position with the 
stereoscopic image and this either under the Rontgen skiagram or 
an ordinary stereoscopic image. These Symphanoplastics will of course 
require a certain amount of technical skill, but if should be possible 
to guide the stylet in this way, so as to shape a model in all respects 
identical with the original object. 
In certain cases it may be of advantage to superimpose a stereo- 
skiagram and an ordinary photographic stereogram. To do this 
effectually, the centre of projection of the Röntgen rays must coincide 
exactly with the centre of projection of the photographic camera. 
This centre may be taken as the optical centre of the lens-system. 
Fig. 8 is a diagram of the Symphanator. A is the anticathode, 
DH the object and P' the Röntgen-plate. 5 is a reflector, making an 
angle of 45° with the principal axis. The photographic camera with 
the lens L is placed at the side, at such a distance that Lm is equal 
to Am. The image on the plate P? is smaller than that at P’, but 
‘corresponds exactly with it in perspective. The Röntgen exposure and 
the photographic exposure may be made simultaneously by making 
the mirror S of a material which allows the passage of the Röntgen 
rays. If preferred one exposure may be taken at a time, by 
using an ordinary mirror, which can be temporarily moved on one 
side. After the first set of stereoscopic negatives are taken, both the 
anticathode and the photographic camera are moved to one side over 
a distance of 65 millimeters. In the figure this displacement is in a 
direction perpendicular to the paper. We thus obtain two stereoscopic 
pairs which will give us an ordinary photographic stereogram and 
a Röntgen stereogram. These may be used in various ways. We may 
magnify the ordinary photographic negatives to the natural size of 
P', and then place, the Röntgen-plates at P, and P, and the photo- 
graphic pictures at P, and P of figure 7. Of course in that case 
the large mirrors S* and S* must be transparent, whereas the small 
mirrors may be opaque. In this way we get the two stereo-images 
superimposed at P so that we may compare one with the other 
with mathematical accuracy. The Röntgen and the pbotographic 
