4. To place the two opposite microscopes 180 apart, set one 

 of them at zero and bring a graduation line to bisect the wire 

 interval. Then the other microscope may be brought to bisection 

 on the opposite line by moving the drum on the screw shaft and by 

 adjusting the comb scale to suit, it may be made to read within a few 

 divisions of the first microscope. Close agreement is not essential, 

 but it is convenient to have both microscopes read the same to 

 the nearest minute. 



Method of Reading 



This will be best understood by considering a special case. 

 Suppose it is required to read the two opposite micrometer micro- 

 scopes of a theodolite whose circle is divided into 10-minute sp.. 



t l' Let 5 revolutions of the screw be equivalent 



to one of these spaces; then one revolution is 

 equivalent to two minutes and the micrometer 

 drums will be assumed to be divided into 60 

 equal partsand numbered from zero to thirty, 

 twice. The relations to be considered are 

 illustrated in the inserted diagram which 

 shows a degree of a circle, the position of 

 the principal points of the microscope object- 

 ive, the position of the micrometer threads, 

 /, /, etc. In this diagram the line be, b'c', is 

 the line defined by the micrometer wires 

 (or the point midway between them) when 

 the micrometer reads zero revolutions and 

 zero divisions. This line falls between the 

 40' and 50' lines of the circle. The reading 

 of the circle is 17 40' plus the distance ab 

 expressed in angular measure. The image- 

 equivalent of ab is a'h', and this is measured 

 by moving the micrometer wires until the 

 space between them is bisected by the 

 image of the 40' line a or by a'. Suppose 

 the distanced' is three revolutions (counted 

 by three depressions of the comb scale) and 

 8.3 sub-divisions 

 of the head. Then 

 / / / / t h e c o m p 1 e t e 



/ / / / readin * i8 17 

 / / 46' 1 



If the op- 



Fig g s posite microm- 



eter reads 197 46' and 11.9 divisions, the mean reading of the circle 

 is (using the degrees indicated in the f'nst microscope), 17 46' 20". 2 

 since ft (8.3 + 11.9) x 2" = 20".2. 



147 



