Whenever an important vertical angle is to be observed, as in 

 stadia topography, the control bubble should be consulted and 

 centered with the Index Adjuster. In some instruments this is 

 accomplished with two capstan screws, but for ready service our 

 construction is such that the bubble can be centered at any time 

 with the small tangent screw operating against a post in the standard. 

 The little check nut is to be first loosened and need not be Used at 

 all thereafter unless the operator chooses to do so. The check nut 

 is used in the original adjustment to clamp the cover guard and 

 vernier scales in a position normal to the rest of the instrument and 

 to prevent insidious errors by accidental interference with the index 

 adjuster. 



The adjustment of the control bubble is contingent upon 

 the seven distinct processes touched upon between pp. 62 and 72. 

 \Vhenthehorizontalcrosswireisproperlycollimatedandthetelescope 

 bubble adjusted to it, the index lines of the vernier scales of the 

 vertical limb must be made to coincide with those of the circle. 



This is accomplished with the little tangent screw called the 

 "index adjuster". It is provided with a check nut so that accidental 

 movement will be prevented. .This accomplished, the control 

 bubble is adjusted to these conditions by simply bringing it to the 

 center of its scale with its own adjusting screws. 



As the lower portion of the instrument departs from ideal con- 

 ditions, for any of the well known reasons, the control bubble will 

 naturally deviate from its normal position. In such a case it is to 

 be re-centered just before any sight involving a vertical angle, by 

 the method previously described. 



The index adjuster is to be utilized continuously for this pur- 

 pose without reference to conditions which prevail in the rest of 

 the instrument. 



Graduations and The Vernier. 



The limb, or principal scale of the portable transit instrument, 

 may not be conveniently divided into spaces that are smaller than 

 1/2 to 1/3, and were it not for the supplementary scale, called the 

 "vernier", readings could not be taken any closer than is possible 

 with the ordinary compass. The device used to read the fractional 

 parts of the subdivision of the limb is named after its inventor, 

 Pierre Vernier, who introduced it in 1631; but it did not become 

 practical until Jesse Ramsden invented the automatic dividing en- 

 gine in 1768. In some foreign countries the scale is still known as 

 the "nonius", after Pedro Nunez whose method of reading the 

 quadrant (1542), however, was totally unlike the scale here con- 

 sidered. 



The vernier may be applied to either curved or straight lines. 

 It consists of a small movable auxiliary scale whose object is to 

 provide a ready means of estimating the fractional parts of the main 



96 



