The Telescope Bubble 



Fig. 49 



Generally speaking, the scientific aspects of the Telescope 

 Bubble and the functions assigned to it have been treated with scant 

 consideration. The question that may rightfully be considered is: 

 Shall the bubble axis be adjusted to the line of sight or the line of 

 sight to the bubble axis? One of these premises must be eminently 

 right, the other emphatically wrong. There is no optional choice, 

 as has been so freely allowed by manufacturers, educators and 

 practitioners. 



One would not attempt to adjust a bubble to parallelism with 

 one of the stadia wires and undertake to do leveling with this com- 

 bination of expedients. In order that we shall be favored with 

 orthoscopic vision at all distances, the horizontal sight-plane must 

 be collimated to the optical axis. The sight line must therefore lie 

 at the basis of adjustments, and the telescope bubble must be pro- 

 vided with mechanical arrangements by which it can be adjusted to 

 the horizontal collimation plane without strain. 



\ 



FifT. so 



When the vial mount is provided with adjusting facilities at 

 only one end, strain is not only likely but very probable. The 

 fixed shoulder at the end nearest the object glass is an unyielding 

 argument against high class construction. The inserted cut, Fig. 50, 



92 



