MANIPULATION OF THE TELESCOPIC ALIDADE 
137 
variable. ‘ It is much greater near the ground than 3 feet above 
it, and generally greater at midday than early in the morning or 
late in the afternoon. The empirical valuation ordinarily 
placed upon the effects of refraction gives the combined for- 
mula: Curvature plus refraction = 0.57135 X D^. 
A table showing corrections based on this formula may be 
found in the ordinary stadia tables. The correction amounts to 
only 0.1 foot for distances of 2200 feet, 0.2 foot for 3125 feet 
and 0.5 foot for 4940 but increases rapidly to more than 5 feet at 
3 miles and 20 feet at 6 miles. It may safely be disregarded for 
the great majority of rod shots,” which will of course be less 
than 3000 feet long. 
The correction is always a minus quantity and should be 
added algebraically after the proper sign has been placed in 
front of the vertical distance as instrumentally determined. It 
will thus increase the vertical distance for angles of depression 
and decrease it for angles of elevation on all fore-sights. Occa- 
sionally, for nearly level sights the correction to be applied for 
curvature and refraction will be greater than the observed dif- 
ference in elevation, and the sign of the vertical distance may 
than be changed. No confusion will arise, if the rule stated in 
the first sentence of this paragraph be rigidly observed. 
ADJUSTMENT OF THE ALIDADE 
The most important adjustments of the miniature or ex- 
plorer’s alidade, which require attention in the field, are those 
for collimation and of the striding level. All other adjustments 
are reasonably permanent as made in the factory. It is, how- 
ever, well for the instrument man to be able to detect, and if 
possible correct, faulty workmanship or damage from mistreat- 
ment or accident. 
Collimation. The line of sight through the telescope is deter- 
mined by the intersection of the cross-hairs, whatever their 
position in the tube, and the nodal point in the objective lens. 
Obviously, the refraction to which reference is here made is not that of the 
direct rays of light from sun or* stars. The. refraction for which correction must 
be made in determining sun azimuth is least between 9 a.m. and 3 p.m. 
