i85
There have to be provisions to compensate, at least approxi
mately, for model deformations caused by the tilt of both aerial
photographs and by an inaccurate orientation under the stereoscope.
Such a model deformation results in the fact that the horizontal
plane is deformed (in first approximation and neglecting the so
called tp-cylinder, caused by the difference of longitudinal tilt of
both photographs) into a ruled surface. To compensate for this, the
string surface itself can be deformed into a ruled surface. This will
be done with the help of four setting screws, situated approximately
at the corners of the useful stereomodel, as these places represent
the average position of selected height control points. (Fig. 2.)
This design allows a fast and simple procedure for the appropriate
setting of the correction surface (see working procedure).
Independant illumination of the surface and the both photo
graphs is provided, the latter being adjustable as to permit the
choice of the optimum viewing conditions for the observer. In order
to overcome a certain difficulty for the observer to fuse stereos-
copically the pictures of identical strips, a smaller number of them
at irregular intervals of each other have been made from different
colour and thickness. The observer has first to concentrate on these
Fig. 2.
