the ed above can be attached to the stereoscope in such a manner that they form a 
C compact unit with it. When the instrument is not in use, the mirrors can be folded 
airy in such a way that the Stereotop I occupies very little space. 
The dimensions of the picture holders 1, 2 (Fig. 7) are 25 X 25 cm® They are 
provided with locating marks for aligning the photographs according to epipolar 
  
ents 
  
the 
ing 
Fig.7. Diagram of "Stereotop III". The correction device for tilt errors of the model is incor- 
ust porated under the r.h. picture holder. 
It 
me 
the rays. À guide bar 3 connects them among each other as well as with a centre board 
ble 4, which may be displaced in the Y direction upon a light-metal base plate 5. Bar 
ial 3 can be displaced in the x direction with reference to the centre board. Finally, 
he the micrometer spindle 6 permits displacing the right-hand picture holder parallac- 
Ly tically by 35 mm with reference to the left-hand picture holder. Parallax p, may 
18: be read to 0,01 mm. The measuring marks (only the left mark 7 is shown) are 
IL, engraved on a transparent plastic ruler which is clamped to the column of the 
11 stereoscope. The cross-slide arrangement is completed by a p, spindle at the r.h. 
picture holder for the elimination of troublesome vertical parallaxes. 
m Finally, the compensation of tilt remains to be described. The basic idea is 
ch very simple. An inclination of the optical model about an axis parallel to the x 
ed axis (tilt) corresponds obviously to a slight modification of the x parallaxes pro- 
re portional to y (the position of the p, micrometer remaining unchanged) or a cor- 
lle responding adjustment of the micrometer, if the pictures remain in their position. 
b- In analogous manner, the equivalent of an inclination about a parallel to the y 
11