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(659) 
out interrupting his observations. This easy method of scrutinizing the spatial 
model keeps the photo-interpreter's concentration intact, thus improving the 
human link in the reconnaissance chain to which Dr. Reyna of the Boston Uni- 
versity Optical Laboratories has recently called attention. 
In order to compensate variations in the base-distance ratio when scanning 
oblique photographs, the left hand field of view can be moved separately in the 
x-direction by turning a small knob, concentric with the knob for the overall 
x-movement of the stereoscopic field. 
A similar small differential movement in the y-direction of the right hand 
field of view enables the interpreter to eliminate occasionally remaining y-paral- 
laxes in the case of a tilt difference between both pictures as is frequently pre- 
sent in aerial photographs made under wartime conditions. This adjustment is 
made by turning a small knob, which is easy to operate, it being concentric with 
the knob for the overall y-movement of the field of view. 
Another unique feature is the facility for permitting two interpreters, each 
of them equipped with an O.D.S. to view the same stereoscope image simultane- 
ously (Fig. 2). To that end the interpreters sit face to face and both stereoscopes 
are set up over the same pair of photographs. This is useful for training purposes 
and for applying the conference system of interpretation. 
A tubular lamp attached underneath the body of the stereoscope provides 
bright illumation for examination of paper prints. Transparencies can be view- 
ed if a light table is available. In the case of colour transparencies, filters can be 
placed over both windows to screen out certain wave lengths of light, a proce- 
dure whose importance has been emphasized by Commander Colwell. 
The eyepieces may be adjusted over a range of 58 to 69 millimeters to ac- 
commodate eyebase differences. They are set at 45 degrees, thus enabling the 
interpreter to work in a comfortable position. 
The O.D.S. can be set up ready to operate very quickly. Its tripod design 
of the legs assures stability and leaves the photographs free for outlining, mea- 
suring and the like. Although the instrument is not intended to be used in the 
field, it may be readily assembled to fit into a compact wooden case. 
Concluding, we believe that this new, imaginative stereoscope, the O.D.S.. 
will enable the photo interpreter to do a still better job.