Instructions for use
Readings are usually taken with the right eye. Owing to differences in the keenness of the sight of the eyes and because of personal preferences the use of the left eye is sometimes easier. It is of prime importance that both eyes are kept open. The sup- porting hand must not obstruct the vision of the other eye.
+ AND - | + AND - |
DEGREE | PER CENT |
SCALE | SCALE |
10 
-
10
-
0 
0
+
10
+
10 
HAIRLINE
EXTENDED
BY OPTICAL
ILLUSION
THIRD SCALE
IN SIDE
WINDOW
Fig. 7. The hairline indicates the reading
The instrument is held in front of the reading eye so that the scale can be read through the eye piece, and the round side-window faces to the left. The instrument is aimed at the object by raising or lowering it until the horizontal hairline is sighted against the point to be measured. The position of the hairline now on the scale is the reading. Owing to an optical illusion the hairline (cross-hair) seems to continue outside the housing and is thus easily observed against the sighted object (Fig. 7).
The left-hand scale angle gives the slope angle in degrees from the horizontal plane at eye level. The right-hand scale gives the height of the point of sight from the same horizontal eye level, and it is expressed in per cent of the horizontal distance. The following example illustrates the procedure.
