THE PRECISE WYE LEVEL 



HE ordinary Y-level has reached the paradoxical 

 distinction of being at once the most popular and 

 most fallible of any topographical instrument, with 

 the possible exception of the surveyors' compass. 



The tendency to increase the length of the 

 telescope involves heavier objective mounts and the 

 re-distribution of weight in the process of focusing 

 has caused one of the most annoying and incurable 

 of the temporal disorders which impede the collateral 

 attempt to secure better results. The longer and larger the tele- 

 scope, the more this difficulty is exaggerated and in this respect the 

 larger models only defeat the purpose for which they are designed.* 

 Inevitable wear in the collars induces a variable factor between 

 the bubble and optical axes and the attempt some years ago to 

 mount the collars on agate pivots only made a bad matter worse; 

 neither has this defect been overcome by the substitution of hardened 

 steel collars which have a tendency to rust under neglect. 



For success and speed too much depends upon perfect vertically 

 in the spindle. If the bubble runs off center when directed to var- 

 ious positions in the field there is no rapid and effectual means, in 

 an ordinary wye with four screw base, to control the bubble at the 

 instant of observation. Strain communicated through the compara- 

 tively coarse adjustment of the leveling screws induces secondary 

 strains in the vertical axis that are aggrivated as the temperature 

 changes. The consequence is that a sluggish bubble that will not 

 indicate disturbances has gone a long way toward making the 

 ordinary wye level "precise" and "stable in adjustments". 



Investigations looking into these various sources of error led 

 J. F. Brander of Augsberg to re-design the lower portion in 1769. t 

 He overcame the necessity for a height adjustment in the wyes, 

 corrected all the difficulties here discussed and secured greater 

 rapidity of manipulation by a "Vertical Control" or elevation screw 

 operating between two pivoted elements of the base-bar. 



Brander first placed the pivots at one end of the instrument. 

 In 1842 Ertel located them at the center of the base-bars as we have 

 them in the model presented on the next page. Since that time no 

 essential modification of these ideas has been introduced until the 

 interior system of focusing was invented to reduce to a negligible 

 minimum errors arising from imperfect balance or temporal derange- 

 ment in the collimation adjustment, and the three screw base 

 (pp. 18 & 139) which provides the most rapid known method of cor- 

 recting slight bubble displacements at the instant of observation. 



* In 1904, Mr. Roman Seelig introduced an exceptional construction that 

 ptfirrved a better equilibrium by moving the objective and eyepiece mounts 

 simultaneously in opposite directions ~>vith u pinion operating in a double rack. 



t See Pacific Builder and Engineer, Seattle. May 31, 1914, p. 337. 



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