58 A MANUAL OF TOPOGRAPHIC METHODS. 



If the wires appear to move in the least, an adjustment is necessary. The 

 eyepiece should always be first adjusted to give distinct vision of the cross 

 wires. This adjustment is entirely independent of all others and requires 

 only that light enough to illuminate the wires enter the telescope or micro- 

 scope tube. This adjustment is dependent on the eve and is in general 

 different for different persons. Hence maladjustment of the eyepiece can 

 not be corrected by moving the cross wires with reference to the objective. 

 Having adjusted the eyepiece, the image of the object observed may be 

 brought into the plane of the cross wires by means of the rack-and-pinion 

 movement of the telescope. A few trials will make the parallax disappear. 



When circles are read by micrometer microscopes it is customary to 

 have them. so adjusted that an even number of revolutions of the screw will 

 carry the wires over the image of a graduation space. If the adjustment 

 is licit perfect, an error of run will be introduced. This may in all cases be 

 made small or negligible, since by means of the independent movements of 

 the whole microscope and the objective with respect to the circle, the image 

 may be given any required size. In making this adjustment some standard 

 space, or space whose error is known, should be used. At least once at each 

 station where angles are read, observations should be made for run of 

 micrometers. An example of such readings is given under sample of field 

 notes below. 



Tangent and micrometer screws should move freely, but never loosely. 

 In making a pointing with the telescope the tangent screw should always 

 move against or push the opposing spring. Likewise, bisections with the 

 micrometer wires should be made always by making- the screw pull the 

 micrometer frame against the opposing spring or springs. 



Extra instrumental errors may lie divided into four classes — namely, 

 errors of observation, errors from twist of tripod or other support, errors 

 from centering, and errors from unsteadiness of the atmosphere. 



Barring blunders or mistakes, the errors of observation are in general 

 relatively small or unimportant. With practiced observers in angular meas- 

 urements, such errors are the least formidable of all the unavoidable errors, 

 and their elimination in the end results is usually well nigh perfect. The 

 recognition of this fact is very important, for observers are prone to attribute 



