COMPENSATION LEVEL 



HK fundamental principle of adjustment which under- 

 lies most instrumental tests is the one of reversion, 

 by which the doubled error can be measured and 

 rectified. It is the simplest and most effective method 

 known and has always dominated the ideals of spec- 

 ulative thinkers on this subject. 



Nearly all the extensive discussion on leveling 

 methods has grown out of attempts to rectify instru- 

 mental deficiences by reducing rather than removing 

 them. In the transit small errors of spacing may be quite success- 

 fully overcome by repetition, but the only method of eliminating 

 instrumental errors in leveling instruments heretofore has been by 

 the equalization of the forward and back-sight. 



This recourse is always tedious and frequently impossible. 

 Every precedent and every impulse have suggested the necessity for 

 a telescope through which telescopic observations might be taken in 

 both directions forward and backward, through the same tube. 

 Such a device is now an accomplished fact, and the principle of 

 reciprocal vision has made it possible to check the sight-line directly 

 against the bubble axis by the simple precaution of turning the 

 telescope end-for-end and applying the eyepiece to the other 

 extremity. 



The adjustment and operation of this self-contained instrument 

 are very simple.* Two achromatic objectives, of equal aperture 

 and focal length, are carefully centered and immovably mounted 

 at opposite ends of the telescope tube. Their optical axes are 

 collimated to one and the same straight line in a special apparatus 

 and the terminations of the optical axis are marked by cross lines, 

 engraved with a diamond point, on the outside surface of each ob- 

 jective. Each objective, in turn, performs the function of a glass 

 diaphragm for the other objective. 



The engraved cross lines supersede the usual spider web and 

 are indestructible. If dust particles collect on the surface of the 

 objective, they will also appear in the held of \ iew and should be 

 dusted off with the camels hair brush provided for this purpose. 

 The cross lines are microscopic and do not interfere in the slightest 

 with telescopic vision. 



The eyepiece is removable and attachable to either end of the 

 telescope and the focusing is accomplished, for sighting in either 

 direction through the instrument, with an interior negative lens 

 which maintains all of the required optical properties. The lens 

 system is symmetrical and the process of focusing provides for 

 homologous positions for the interior lens. 



* Sff Ar licit by Dunbar D. Srotl, Vol. A.V.V/7, Trans. A. S. C. K. /?/-?, 

 p. 1172 ', etc. A copy of this paper ivtll bf imul<d <i leyutit. 



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