COMSTOCK STUDIES IN ASTRONOMY 71 



For this purpose the instrument should be carefully lev- 

 elled at the beginning of the work and the telescope di- 

 rected at some object, approximately at right angles to the 

 line joining two of the leveling screws of the instrument. 

 Let the zenith distance, ', of this object be determined 

 from readings of the vertical circle taken Circle Right and 

 Circle Left. The vertical axis is now to be deflected 

 toward the object by turning the leveling screws, and the 

 zenith distance of the object, reckoned from the vertical 

 axis of the instrument, z", is to be determined from circle 

 readings in the same manner as z'. We then have, ob- 

 viously, 



y = z' - z" 



To make sure that the deflection of the axis lies in the 

 plane passing through the object sighted upon, it is well 

 to note the position of the bubble of that level of the in- 

 strument which is at right angles to the telescope tube. 

 The leveling screws must be so turned that the reading of 

 the bubble of this level on its scale is approximately the 

 same after deflection as before. 



By comparison with micrometric apparatus, this deter- 

 mination of y and the resulting value of d may seem crude, 

 but with a vertical circle reading to minutes only, the 

 values of z' and z" can be determined within 30", and if y 

 be made 3, d will be determined with a probable error of 

 one part in four hundred, an accuracy quite sufficient for 

 even the most delicate level. The value of y should be 

 between 1 and 3, a coarse vertical circle and fine hori- 

 zontal circle corresponding to the larger limit, and the 

 reverse conditions to the smaller one. 



To illustrate the method, I select the following partial 

 investigation of the microscope level of a small universal 

 instrument, Bamberg No. 2598. The level was investigated 

 by means of the circles of the instrument to which it was 

 attached, without removing or in any way disturbing it: 



