STANDARD - Survey Instrument Antique Center!

21
applied with an adjusting pin about one and one half inches long, and held between
the thumb and forefinger, will then make a perfect contact. For instance, after the
it is
opposing capstan-heade I screws of the cross-wire reticule have come to a bearing,
only necessary to give them each a slight turn, say from 20 to 30 (with the usual pitch
of these screws) in order to insure such a tightness that a moderate pressure of the
finger upon these screws, or an accidental gliding of the hand over them, cannot change
their relative position. On the other hand, if one pair of these opposing screws be
in all likeli-
fastened tightly during the tentative process of adjustment, there will be,
hood, at the end, an excessive strain exerted upon the pair of opposing screws at right
angles, which will make itself felt at any change of temperature, or whenever any
external pressure may be momentarily applied to them. Jt is but natural that these
continual changes in the resultant pressure must affect the adjustments in a like man-
ner. To obviate such changes the procedure should be as follows :
Having placed approximately in position the principal wire of an instrument : viz.,
in a transit, the vertical wire in a plane perpendicular to the horizontal axis of revolu-
tion, in a level, the horizontal wire in a plane perpendicular to the vertical axis
of revolution, the other wire should be approximately adjusted for collimation,
with the capstan-headed screws only moderately tightened. This accomplished, the
capstan -headed screws of each pair in succession should be unscrewed about one-
quarter turn, and again screwed tight the same amount. Now if the two pairs of
opposing screws have exerted no undue strain upon themselves, the telescope tube, or the
wire reticule, the principal wire will still be in the perpendicular plane ; but if the screws
have been used too much the wire will have slightly moved out of the perpendicular
plane. Therefore all four capstan-headed screws will have to be released again, say
about | turn, so that they may be moved simultaneously until the principal wire is
again in a plane perpendicular to the axis of revolution, and then each pair in succession
must be again tightened an equal amount. The adjustment of the wires lor collimation
must now be made in turn the less important wire should always be taken
up first by slightly releasing the capstan-headed screw away 1 from which the wire
must be moved, and tightening the opposite screw the same amount, and repeating
this process until the adjustment is gradually perfected. If during this operation either
or both of these wires Lave become so much displaced that the capstan-headed screws
have to be moved more than a quarter turn, it would be advisable to slightly release all
four of them again, in succession, and commence anew.
It should be said here, that the force applied by the capstan-headed screws
cannot break or affect the tightness of the wires in any case, since the reticule, as
made by us, although very light in weight, is of a very stiff form. Too great pressure
exerted by the capstan-headed screws against the outer tube of the telescope may,
however, change die form of the main tube, thereby affecting the true fitting of the
object-slide, and creating friction of so serious a nature as to lead to the fretting of the
object slide mentioned in other paragraphs.
In following the above-described course, the cross-wire reticule occupies a position
in the telescope free from any excessive side strain the result of which is found in the
;
greater permanency of these adjustments and ; although it may require a little more time
for an inexperienced person to make the adjustments in this manner, the satisfaction
derived from their greater permanency will more than recompense for the extra time
spent on them. The adjustments should be made at leisure, and should not be
meddled with, unless they appear to be permanently deranged; when, ordinarily,
the adjustments will merely require a very slight turn of the capstan-headed screws
and opposing nuts in the proper direction. 2
Unequal exposure of the instrument to the
sun, or exposure to sudden changes of temperature, may for a time expand some
parts more than others, so that the instrument may seem to be slightly out of adjustment.
In such a case it would be better to stop temporarily and cover the
instrument with a bag to allow the temperature to become equalized, instead of attempting
adjustments that would need to be repeated when the instrument is
again in a normal condition. The use of metals of different co-efficients of expansion
in the construction of corresponding parts of an instrument w?ll naturally lead to a
1 We refer here exclusively to the more common instruments of American manufacture, where the
shoulders of the capstan-headed screws bear against ihe outer tube of the telescope, and where the adjusting
threads are contained in the wire reticule. In other designs where, as in most instruments of Cont nenta
>
Europe, the capstan-headed screws are made to butt ajrainst the wire reticule, the capstan-headed screws
towards which the wire mu-t be moved, must first be loosened. In the latter case this action is identical
with that of opposing nuts used for the adjustment of most telescope levels on American instruments.
2 See page 58.