The slope-angle motion is controlled by knob D or D'. By adjusting 
either knob the targets are simultaneously set at any desired angle of slope. The 
amount of the angle of slope is read on a protractor-like dial mounted on each 
shaft. The dials are calibrated in units of |® and range from 0°, which is parallel 
to the base of the instrument, to 90°, which is perpendicular to the base of the 
instrument, 
The Instrument is equipped with six pairs of targets, of which only one 
pair Is used at a time. The targets are of different sizes and should be selected in 
accordance with the surface expression of the slope to be measured as it appears in 
the stereoscopic model. The target fits into a slot cut in the end of the shaft so 
that the target pairs are easily interchangeable. The particular targets shown In 
figure | were designed for photogeologic work. 
SUPPLEMENTARY SLOPE MODEL 
The Supplementary Slope Model (fig. 2) Is a line drawing in two parts. 
When the two parts are viewed stereoscopically, two sloping rectangular planes 
are visualized In space. 
The parallactic displacement, and thus the steepness of slope is fixed 
for a selected focal length and photo base. However, for other focal lengths and 
photo bases this parallactic displacement represents different known angles of slope. 
These different angles of slope are shown In the table that is a part of the Supplementary 
Slope Model. This table covers only 6- and 8}-inch focal length photography with 
photo bases from 2.5 to 4.5 inches.