ternational Congress of Photogrammetry, Paris 1934), with a hint to De- | not ] 
  
  
ville's disposition. | Devi 
In the present instrument though one cannot talk any more about 
a Deville system; according to this the photographs are in fact disposed the | 
one in front of the other, and before them are placed two semi-transpa- 
rent mirrors at 45°, in an invariable position with respect to the camera | 
axis, with the purpose of taking to the observer’s eyes the image of both | 
photograms and the same time to let a mark pass through the same | 
semi-transparent mirrors, connected with a tracing pencil. In the new 
Phctostereograph is applied for the first time the principle according te | 
which the fusion of the image emerging from the photogram, and of the | 
  
  
  
  
Fig. 1 — General diagramm. 
mark takes place by means of parallel rays, before they enter the optical | s 
observation train. | t 
Therefore in the two cameras of the instrument under examination 
— reciprocally oriented, and which may have or not optics similar to the | I 
taking ones — can be identified the homologous directions of the various - 
points of the photographs, by aligning with them the telescopic marks, 1 
contained within steel collimating rods, which materialize the directions 
of the homologous rays. 
4