There exists a popular and erroneous impression that the 

 longer the telescope, the more accurate the sight. By the same 

 course of reasoning, a more accurate adjustment can be perfected 

 over a test line 200 ft. long than in the space between the collars. 

 The first proposition is true only in a relative and limited sense, but 

 the second is too obvious for discussion. 



There also exists an even more erroneous impression that the 

 longer the level vial, the greater the sensibility. This is without 

 foundation in fact. The sensibility of a spirit level depends only 

 upon the radius of curvature of the inner surface of the vial. 



ADJUSTMENTS 



If the vertical axis is chosen as a basis of adjustments, the 

 bubble is adjusted to it with the "vertical control" by 180 revolu- 

 tions, and the bubble need not therefore be provided with separate 

 adjusting nuts; but if the sight-line is adopted as a basis, as recom- 

 mended and urged by us, adjusting nuts for the vial tube are neces- 

 sary and are therefore furnished with each instrument. 



The sight-line, as carefully collimated to the optical axis before 

 leaving our works, by the method described on page 18, will be 

 found to maintain its position with great persistance. In view of 

 this fact the adjusting screws for the diaphragm are concealed be- 

 neath a threaded ferrule, as shown in the illustration. 



Conduct the adjustment by either of the methods described 

 for the dumpy level, and in taking the final step bring the horizontal 

 wire to read on the target, as set, by use of the vertical control. 

 If the bubble runs off center, correct the entire error with the ad- 

 justing nuts at the end of the vial tube. 



By reference to the inserted diagram the sight-line, SL, and the 

 bubble horizon, HO, are now parallel. Figure 12 will further 



Vig. 12 



show how the sight-line can be brought into the true hori/nn by 

 use of the vertical control, C, without regard to the adjustment of 

 the vertical axis, VX. 



22 



