[DEVILLE] WHEATSTONE STEREOSCOPE IN SURVEYING 67 
an inclination equal to the slope of the line joining the stations 
from which the views were taken, which slope has to be measured on 
the ground either by direct angular measurement or by ascertaining 
the difference of altitude between the stations. The horizon and 
principal lines of each transparency must now be adjusted. Com- 
mencing, for instance, with the transparency on the left side which 
we may assume to have been taken from the highest station, the 
screen of the tracer is set at the height of the left eye hole; this is 
done by setting the screen at a height above zero equal to half the 
difference in altitude between the two stations (reduced to scale). 
The tracer is moved until seen through the eye hole it appears close 
to the edge of the frame; by means of the screws H H, the frame is 
displaced up or down until the hole of the screen is bisected by the 
end of the horizon line. The tracer is now moved until it is seen 
close to the opposite edge of the frame which is adjusted in the same 
manner. The same operation is repeated several times until both 
ends of the horizon line are in correct position. 
To adjust the principal line, the trace of the vertical plane 
parallel to the axis and passing through the eye hole is drawn upon 
the table and the point of the tracer pencil is set upon this trace. 
The screen of the tracer is moved up or down until close to the 
upper or lower edge of the transparency frame and fixed in that 
position. The frame is now moved by means of the screws J J until 
the hole in the screen is bisected by the extremity of the principal 
line. Should the screen be moved until seen near the opposite edge 
of the frame, the hole must be bisected by the other extremity of 
the principal line, without further adjustment. Each transparency 
is, of course, adjusted in the same manner, and by a separate operation. 
Looking now through the eye holes with both eyes, the hole in 
the screen appears as a bright point projected over the relief of the 
ground. Guiding the tracer by hand so that the point seems to 
follow and remain in contact with the surface of the ground, the 
pencil describes on the table a contour line. When the screen is at 
zero, the altitude of the contour is equal to the mean of the altitudes 
of the two stations. For describing any other contour line, the 
screen is displaced above or below zero a distance (reduced to scale) 
equal to the difference between the mean altitude of the stations 
and the altitude of the contour line to be drawn. Other lines not 
situated in horizontal planes, like the shore of a rapid river, a road 
or other features of the ground, are drawn by using a screen with a 
vertical slit instead of a hole. The tracer is moved so that the slit 
appears to pierce the ground along the line which is to be plotted. 
