equal amount to the left, the test will show that, while the 

 plane table is not horizontal in the direction of the telescope 

 axis, the axis itself is correct. 



The standards are cast in bronze and the axis in No. 1 red 

 metal so that fretting is reduced to a minimum. The adjust- 

 ment as made in the factory, therefore, may be considered as 

 reasonably permanent. With the vertical arc on one extension 

 of the horizontal axis and the vertical clamp on the other, it 

 would be difficult to provide an adjusting block, especially when 

 the most experienced operators have found such a contrivance 

 unnecessary. 



Collimation for both of the cross wires is tested and rectified 

 as in the Y-level, as directed on p. 6. The telescope is 

 mounted revolvably between 180 stops in the axis-sleeve for 

 this purpose. At the under side of the sleeve is a plunger ex- 

 erting just enough pressure to prevent the telescope from turn- 

 ing on its axis in the process of focusing. All alidades are made, 

 by common consent, with the inverting telescope only. If the 

 cross wire has to be moved in making this test, it should there- 

 fore be moved in the direction which is apparently necessary. 



It was thought heretofore that if the vertical cross wire were 

 brought into the apparent center of the field, the practical re- 

 quirements in the case had been fulfilled. No telescope, 

 however, is capable of accurate vision, in both planes, at all 

 distances in the field, until both wires are properly collimated. 

 With all the conveniences at hand, there is no excuse why the 

 vertical wire should not also be accurately located in the optical 

 axis by the process of rotation as directed above. 



The Striding Level, which is removable and reversable, is sup- 

 ported on two red metal collars that do not constitute the 

 axis of longitudinal revolution, as in a Y-level. When instru- 

 ments leave our works, however, they are trued up to the same 

 axis of rotation. There is very little chance for wear in the 

 carefully protected rotation sleeve and the collars are subject to 

 little or no wear, so that we may reasonably assume that this 

 adjustment will con temporize itself with the life of the instrument. 

 The rapid test therefore is to level the telescope by the strid- 

 ing level, then turn the striding level end-for-end on thecollars. 

 If an error is indicated, correct one half with the tangent screw 

 and the other half in the striding level. To accomplish this, 

 turn the set screw, in the crotch of one of the wyes, with a 

 driver. As in the wye level this secures parallelism between 

 the bubble axis and the contact points on the collars, but does 



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