MANIPULATION OF THE TELESCOPIC ALIDADE 
135 
mine the corresponding difference in elevation for each Stebinger 
unit as before; with the factors thus obtained plot a curve using 
the numbers of divisions as the abscissas and the values as the 
ordinates. From this curve the point will be readily apparent 
at which the micrometer screw begins to work with reasonable 
uniformity; begin constructing the table to be used at that 
point. From the curve determine the points where the factors 
for two successive divisions differ by 0.00005 and compute the 
factors for the number of divisions represented by each of these 
points; interpolate between the values thus obtained to complete 
the table. 
The table of Stebinger factors should be checked every few 
weeks by comparing a half dozen Stebinger readings, made at 
haphazard intervals well distributed throughout the range of 
the micrometer screw, with the corresponding readings of the 
vertical arc. The Stebinger factor should be identical, on the 
average, with the vertical distance corresponding to the arc 
reading. 
In reading the Stebinger drum the observation should be made 
from directly above the celluloid or steel index so as to project 
the index line vertically downward to the drum. Ordinarily, a 
reading to the nearest Stebinger division is sufficiently close, but 
for low angles and long ^ ^shots’’ it is better to estimate half 
divisions, and for the nearly horizontal two-to-five mile ^ ^shots’’ 
of triangulation it is frequently worth while to estimate to tenths 
of a division. For these long sights, the distance of which is 
determined by scaling off the space on the map, there is a theo- 
retical error in using tables based on the formula involving one- 
half the sine of twice the angle, but there is practically no dis- 
crepancy here for the difference between the sine of a small 
angle and one half the sine of twice the angle is negligible. 
CURVATUEE AND REFRACTION. 
No matter what method of determining vertical distances is 
used, a correction for curvature and refraction must be applied 
to all shots’’ of a mile or more in length. The level datum to 
