physiography of the terrain. For example, in areas where the direction 
of prominent ridges and valleys is nearly uniform, the scanning operation 
is facilitated when performed in a direction parallel to these major 
features. Provision for adjusting the scanning direction has been included 
in the new instrument. The supporting bar for ER-55 projectors may be 
rotated about a vertical axis to permit a deviation between the direction 
of scan and the air base or flight direction. 
The most striking modification in the new design is the replace- 
ment of the flat film-supporting platform with a 12-inch diameter cylinder. 
This cylinder, around which the blue-sensitive film is attached, is 32 
inches long and is enclosed in a light-tight outer drum. The cylinder and 
its concentric drum-shaped envelope are alined with their longitudinal 
axis horizontal and parallel to the X-direction of the instrument. The 
platen carriage containing the scanning aperture is mounted on a track 
that extends the length of the outer drum. The track is fixed to the top 
surface of the outer drum, permitting the scanning slit to traverse the 
length of the inner cylinder. The entire assembly of the drums, platen 
carriage, and track can be raised or lowered as a unit by the operation 
of a manual control. The emulsion of the film is exposed, through the 
scanning aperture, to small differential areas of the stereoscopic model 
surface as the platen carriage moves along its track. Although the film 
is exposed to both the red and blue projected rays which create the 
anaglyphic model, only the blue rays have an actinic effect on the film, 
The travel direction of the platen carriage is alternately from left to 
right and right to left; the reversal occurring after the completion of 
each scan. 
Two automatic, synchronized motions take place when the end of 
a scanning path is reached. The inner cylinder is rotated about its 
longitudinal axis, and the assembly of the drums and platen carriage is 
translated in the y-direction. The amount of cylinder rotation is pre- 
cisely that necessary to place the unexposed section of film immediately 
adjacent to the film area previously exposed into position under the path 
of the scanning slit. At the same time, the assembly is translated the 
distance necessary to scan a path across the stereoscopic model adjacent 
to the previous path. The synchronized rotation and translation must 
therefore be set to agree with the width of the scanning path. Adjustments 
are provided for scan widths ranging from 1.0 mm. to 24.0 mm. 
Several important advantages accrue from this drastic change in 
design. It is generally easier, and therefore cheaper, to machine a 
cylindrical surface to a specified tolerance than to machine a flat surface 
to the same tolerance. Those who have had experience in attempting to 
force photographic film to lie in a flat plane will appreciate the fact 
that it is much easier to place film around and in contact with a cylin- 
drical form than to cajole, plead, beg, or tape the film to remain in 
contact with a flat surface. 
The more important advantages anticipated as a result of this 
change in design are operational. The operator will be in a comfortable 
seated position during the entire scanning operation. By moving his chair 
forward as the cylinder is translated from its starting position at the 
front side of the instrument toward the rear of the instrument, the operator 
will have little difficulty in maintaining an advantageous and nearly 
constant viewing angle and distance to the scanning aperture. These gains 
are significant because the operator formerly stood bent over during the 
scanning procedures; moreover, because of the restrictions imposed by the 
physical size of the flat film platform, the viewing distance to the rear 
portions of the stereomodel was excessive. 
5