MANIPULATION OF THE TELESCOPIC ALIDADE 
99 
cross-hairs. These lenses are contained in a separate smaller 
tube which may be slid in or out of the telescope tube by means 
of a rack and pinion turned by the focusing screw oii one side of 
the telescope in front of the transverse axis. This is necessary 
to define sharply the image of distant or near objects at will, or 
in other words to provide a means of bringing the objects in the 
field of vision into focus. 
The eyepiece is similar to a microscope; its purpose is to mag- 
nify the cross-hairs and the image thrown by the object-glass. 
It may be adjusted to suit the eye of the individual observer by 
twisting the knurled ring at the end of the telescope tube. This 
Fig. 1. The Gale Alidade with Stebinger Gradienter Drum Attachment 
AND Detachable Parallel Rule; Striding Level in Place and 
Compass Needle Released, (Photograph from 
U. S. Geological Survey.) 
shifts the eyepiece toward or away from the cross-hairs. Once 
properly adjusted there is no reason for changing the position of 
the eyepiece unless the instrument is used by another observer. 
If the image formed by the object-glass is not in the same plane with 
the cross-hairs, any movement of the eye is likely to cause an appar- 
ent movement of the image with respect to the cross-hairs. This is 
called 'parallax. The effect is similar to that produced in looking 
through a window, where any movement of the eye causes an appar- 
ent movement of objects outside. Parallax may render accurate work 
impossible. To remedy it the image and the cross-hairs must be 
brought into the same plane. Two steps are necessary. 
