the telescope the same way and so prevent the spider webs from sagging 

 or wrinkling in damp weather. If, in the course of time the telescope 

 lenses seem to loose their lustre, unscrew the objective, rack out the 

 focusing mechanism, remove focusing pinion if necessary and un- 

 screw the inner lens barrel. Dust particles should be removed with 

 a camels-hair brush, but dirt is best removed by a soft rag moistened 

 with gasolene and dried w ith soft linen. 



Whenever a lens system is disturbed, however, both the collima- 

 tion adjustment and the stadia interval will most likely be affected. 

 Take, for instance, an objective in one of our tachymeters in com, 

 bination with the focusing lens which has an equivalent focal length 

 of say 176 mm. The space between the stadia wires must be 1.76 

 mm. When it is understood that these intervals must be figured 

 to the nearest Moo mm, one can appreciate the delicacy required i n 

 mounting. 



Alignment and Eccentricity 



The circle is provided with opposite verniers to correct for 

 small errors in the spacing thereof, but the index line of each vernier 

 should properly lie in a straight line through the center of the 

 azimuth axis. This setting is accomplished by the maker with the 

 aid of two diametrically opposite microscopes and is generally 

 conceded to be both accurate and permanent. The mechanical test 

 of this setting depends upon the accuracy of spacing between the 

 graduation lines and shows only a probable error, whatever the size. 

 The test for eccentricity of setting is developed in Merriman-Brooks 

 Handbook, begining at p. 82. 



Obviously there would be two opposite positions, that will 

 never show eccentricity, when the line passing through the zeros of 

 the verniers also passes through the center of the azimuth axis. We 

 keep the errors of eccentricity reduced toafew seconds of arc. This is 

 a matter that need not seriously concern the average surveyor be- 

 cause errors of this size will not be distinguishable or effective in 

 circles graduated to read minutes of arc. It is recorded in Johnson- 

 Smith at p. 81 that the error of eccentricity involves no appreciable 

 error in measuring horizontal angles. An eccentricity of Mooo of 

 an inch in the azimuth axis will cause a maximum error of 1' 08" 

 on a 6-inch circle if but one vernier were read; but a mean ot botn 

 verniers eliminates all errors due to this cause. 



Fortunately for the better class of field work, as well as for 

 precise triangulation, much of the effect of this recognized source of 

 error can be eliminated by the well known process of repetition 

 a certain number of times with the telescope erect and an equal 

 number of additional observations w ith the telescope inverted. 



The accuracy of spacing, centering and alignment having been 

 established for any particular make of transit, most engineers will 



Hi 1 



