The new method and instrument 
Fig. 1 shows photographs 1 to 5 of a strip of aerial photographs. 
Fig. 2 shows how the photographs are arranged for observation under a conventional mirror 
stereoscope; A, B, C and D show the zones — each separated — which can be seen stereos- 
copically. 
In Fig. 2 the lines along which the pictures, according to my idea, are first to be cut are 
shown dotted and the corresponding halves of the pictures are identified with the conse- 
cutive picture number by the letters a and b. The picture halves are now arranged according 
to their consecutive numbers, as shown in Fig. 3 in two parallel strips. The upper strip 15 | 
comprises the halves a and the last picture and the lower strip 16 comprises the first 
picture and the halves b. Thus all the parts which must be seen by the left eye for stereo- 
scopic perception are arranged in the lower strip while all the parts which must be seen by e e 
the right eye are arranged in the upper strip. Fig. 3 also shows how the two strips are 
displaced relative to each other in the direction of the flight-strip axis by the eyebase 6 
in order to accommodate the stereoscope to be described later. 
The entire strip can now be observed stereoscopically, for example with a modified 
Helmholtz type of mirror stereoscope. The eyebase 6 is set parallel to the strip axis. 
Fig. 4a shows a plan of the stereoscope and Fig. 4b the side view. The mirrors 10 and 11 
below the oculars 8 and 9 deflect the rays through 909 on to the further mirrors 12 and 13. 
The rays are then deflected on to the picture plane 14. The upper strip is at 15 and the 
lower strip at 16. The modification of the Helmholtz type of mirror stereoscope is shown 
in Fig. 4a. The two mirrors 10 and 12 must rotate the optical axis through the left ocular 
by 90? in a anti clockwise direction so that the observer 7 sees the lower strip 16 with the 
left eye. The mirrors 11, 13 must rotate the optical axis through the right ocular 9 by 90° e & 
in the anticlockwise direction so that the observer 7 sees the upper strip 15 with the right 
eye. Therefore it is a necessary condition for the stereo effect that homologous points of 
the two strips 15 and 16 must be displaced relative to each other by the eyebase 6 in the 
direction of the flight-strip axis. 
The strips 15 and 16 are mounted on a slide which can be displaced along the axis of the 
flight-strip so that the observer can scan from one end to the other. 
If a mirror stereoscope modified in this way is used for observation of aerial photographs 
of 23 cm x 23 cm format, the most widely used format, to scan the whole length of the | 
strip then correspondingly large mirrors will be necessary. Furthermore, as a result of the 
large separation there will be an optical reduction of scale and the observing position will 
be uncomfortable.