Watch the telescope bubble and estimate the deviation in seconds 

 of arc on the scale. If the bubble moves a third of a division on 

 a 30-sec. scale during a complete revolution of the underplate, a 

 total eccentricity of 10 seconds in the centers may be assumed. 

 This will mean a plus or minus error of 5 seconds, which is just 

 as likely to be compensating as cumulative in running levels. 



The plate bubbles, as a matter of precedent and not as a matter 

 of reason, usually have a sensitiveness of about 60 seconds per 

 division. Some plate bubbles are less sensitive than this so that an 

 eccentricity of 10 seconds in the centers will scarcely be noticeable, 

 except when tested with the telescope bubble as explained above. 



An error of this size is not worth taking into account as to its 

 effect on reading horizontal angles, particularly when the prob- 

 ability of much more formidable errors in linear measurement is 

 considered; but for leveling purposes it will be apparent that the 

 accuracy of the operation is not so much dependent upon the length 

 of the spindle as upon the care with which it is centered. 



We insert a table of errors calculated on a basis of 1000 

 let this be feet, or meters, or any unit of measure. The errors are 

 given in percentages and will be proportional for other lengths of 

 line. 



By reference to the table it will appear that an error of 30 

 seconds in the centers will produce a probable plus or minus error 

 of .073 per 1000 which scarcely comes within the limits prescribed 

 for ordinary good leveling. Let it be borne in mind, however, that 

 the effect of such an error upon reading minute graduations need 

 not be so seriously contemplated. 



III. Parallax. 



The function of the objective is to produce a minute aeria 

 image of the field at the plane of the cross wires, and the coincidence 

 of the cross wires with this microscopic image depends upon the 

 accuracy with which the objective is focused. 



The office of the eyepiece is merely to magnify the image of 

 the field so as to produce a visual image some twenty to thirty 

 times its original size. These conditions make it first necessary to 



64 



