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REPORT OF COMMISSION VII 9 
properly positioned for stereoscopic study. They also provide a secure attach- 
ment of the lenses to the board. Accordingly, the interpreter in the field can ex- 
amine the photos stereoscopically while in a standing position much more easily 
than with the various types of pocket stereoscopes. 
B. STEREOSCOPES FOR OFFICE USE: 
Virtually all *arm-chair" photo-interpretation is still performed by examin- 
ing paper prints with either a mirror-type or lens pocket-type of stereoscope. 
The fact that no improved model has been widely accepted probably is ex- 
plained by the fact that the information obtainable through use of existing 
equipment generally has proved to be adequate for meeting certain minimum 
requirements. However, this does not preclude the possibility that much addi- 
tional information of value might be interpreted by viewing the photographs 
through an improved type of stereoscope. Such a stereoscope might be equipped 
to provide superior illumination of the photographic image and to offer a wide 
range of magnifications by means of a compound lens system. 
For certain types of photo-interpretation there is urgent need at present for 
development of a stereoscope embodying several, if not all of the following 
features: : 
(1) Facility for viewing transparencies by transmitted light, in lieu of the present 
system of viewing opaque prints by reflected light. It has long been known that 
the former provides much finer tone gradations than the latter, thereby facili- 
tating interpretation. 
(2) Facility for projecting the stereo image onto a frosted screen (from a point be- 
hind the screen) in such a way that by changing the distance between the 
projector and the screen, the scale of the projected image could be varied be- 
tween wide limits. By using the polaroid principle of stereo projection, not only 
monochromatic transparencies could be viewed, but color transparencies as 
well. 
(3) Facility for independently adjusting the light intensity from either element of the 
stereo projector. Thus, if the image received by the interpreter’s left eye were 
less bright than that received by his right eye, he could increase the intensity 
of illumination of the weaker image until both left and right eye images were 
perceived with equal intensity. This would permit the photo-interpreter to 
achieve the maximum stereoscopic effect. Furthermore, such a lighting arrange- 
ment would permit the interpreter to increase the light intensity through both 
projectors simultaneously when he wished to perceive the maximum detail 
within shadows or in otherwise underexposed portions of the transparency. 
(4) Facility for viewing the projected stereo images of color transparencies through 
various filters which might be placed either over the objectives of the projec- 
tors, or over the eyes of the interpreter. This would permit full exploitation of 
the fact that one portion of the visible spectrum provides better tone contrast 
for one group of objects while a different portion of the spectrum may be supe- 
rior for a second group of objects, etc., as previously mentioned. 
(5) Facility for permitting two or more interpreters to view the same stereoscopic 
image simultaneously thereby enabling them to use the so-called “conference” 
system of photo-interpretation so useful in the training of photo-interpreters 
and in the interpretation of certain features which require discussion by several 
types of specialists. 
C. PHOTO-INTERPRETATION KEYS: 
The great interest that interpreters have shown recently for the development 
and use of photo-interpretation keys constitutes one of the most significant 
developments in the field of photo-interpretation since the 1948 World Congress. 
In the United States, a special committee known as the Interservice Committee