140 
KIRTLEY F. MATHER 
ratio of 1 : 100 between observed intercept on the rod and distance 
from alidade to rod. Read the rod intercept at accurately 
measured distances between 100 and 1000 feet from the instru- 
ment. If the stadia hairs do not give intercepts sufficiently 
accurate for the work in hand, the discrepancy may be remedied 
either by preparing a specially graduated rod adapted for the 
particular alidade or by computing a constant by which observed 
distances must be multiplied in order to give true distances. 
Bullseye level. The inner surface of the circular bubble by 
means of which the alidade base is made approximately to coin- 
cide with the horizon is that of a sphere of long radius. The 
alidade base should be parallel to a plane which is tangential to 
this sphere at the point defined by the bubble indices. Once 
adjusted in the factory it is rarely necessary to rectify the bubble 
to keep this parajlelism within the rather broad limits required 
for plane table work, but in case of this necessity place the ali- 
dade upon a plane surface which is known to be level in all direc- 
tions and tighten the screw toward which the bubble seems to 
creep.. 
Telescope axis. In order that the vertical cross-hair shall 
travel in a vertical plane, the telescope axis must be adjusted to 
horizontality. The requirements for plane table work are suffi- 
ciently met by the plumb line test. Carefully level the plane 
table and place the alidade along a ruled mark. Hang a plumb 
line in the field of view and revolve the table to check against it. 
If the vertical hair deviates to the right for instance, reverse the 
alidade along the guide line and test on another plumb line 
swung in the new field of view. If in this case the vertical hair 
deviates an equal amount to the left, the test will show that 
while the plane table is not horizontal in the direction of the 
telescope axis, the axis itself is correct. 
Adjustment of the horizontal axis, should this ever become 
necessary, cannot be made in the field. The factory adjust- 
ment is considered to be so permanent that an adjusting block is 
not provided on alidades. Moreover, it would be difficult to fit 
such a contrivance, for the vertical arc is on one extension of the 
horizontal axis and the vertical clamp is on the other. 
