100 
KIRTLEY F. MATHER 
(1) Point the telescope toward the sky and move the eye-piece in 
or out until the cross-hairs are as well defined as possible, i.e., in per- 
fect focus 
(2) Direct the telescope to the object and focus the object-glass as 
usual, keeping the eye on the cross-hairs until the image appears in 
sharp focus. Test by moving the eye from side to side, and if neces- 
sary move the object-glass slightly until parallax disappears. 
The more accurate the work the more care should be used to elimi- 
nate parallax, while the higher the power of the telescope the more 
difficult it is to do this.^ 
At the ocular end of the eyepiece is a fixed prism which deflects 
the rays of light at right angles to their line of passage through 
the telescope and at the same time produces an erect image of 
the field. All observations are to be made while the operator is 
Fig. 2. Diagrammatic Longitudinal Section of Alidade Telescope; 
Paths of Light Rays Indicated by Broken Lines 
looking directly down into the eyepiece prism. Some alidades 
are fitted with a periscope’’ or tubular roof above the eyepiece 
prism in which is housed a lens for the reversal of the image. 
When this is wanting, images in the field appear right side up 
but reversed from right to left. To keep maximum illumination 
of an undistorted field as observed through a roofed prism entails 
precision grinding of the highest order of merit; this is obviously 
very expensive so that there exists some doubt as to whether it 
actually pays to correct the relation of the elements of the field 
in this particular. 
The magnification of miniature alidades is ordinarily 16 or 
20 diameters. 
® J. C. Tracy, Plane Surveying. John Wiley and Sons, New York, 1907, pp. 
555-6. 
