25 



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 

 opposing capstan-headed screws of the cross-wire reticule have come to a bearing, it is 

 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 

 fastened tightly during the tentative process of adjustment, there will be, in all likeli- 

 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. It 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 succes- 

 sion must be again tightened an equal amount. The adjustment of the wires for colli- 

 mation 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 have 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 ad- 

 justment. 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 at- 

 tempting 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 will 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 the outer tube of the telescope, and where the adjustin g 

 threads are contained in the wire reticule. In other designs where, as in most instruments of Continenta > 

 Europe, the capstan-headed screws are made to butt against the wire reticule, the capstan-headed screws 

 towards which the wire must 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. 



1 See foot-note on page 49. 



