342 



Transactions of the Society. 



diagram A (fig. 2) ; in diagram B is shown a slight alteration to 

 the indices which entirely avoids this difficulty. An alternative 

 form which would allow greater accuracy is shown at C. The 

 method of using this form of index is as follows : — 



The short graduated arm is so constructed that the angle it 

 makes may be altered without changing the position of the zei'o. 

 By means of a preliminary measurement the approximate K.A. 

 of the objective is obtained. Suppose it to lie between 0-8 and 

 • 9, the index is placed so that the zero corresponds to 0*8, and 

 the graduated arm is now gently rotated till the 10 graduation 

 corresponds to • 9. The intermediate graduations now correspond 

 to the decimal subdivisions between • 8 and • 9, and may be 

 used for reading to approximately 0'003 N.A. It should be noted 

 that for the scale on the short arm to be uniform its edge must be 

 the segment of a circle. This circle has nearly the same radius 

 as that of the glass plate. 



A. 



B. 



C 



Fig. 2. — Diagrams to show the present shape of the indices A and 

 two improved types, B and C. 



At A the subdivisions between the graduations are difficult to estimate 



by the eye, because only one side of the index is visible. 

 At B the index has been cut away, thus facilitating the correct 



estimation of the interval- 

 At C the index is fitted with a small scale of adjustable inclination, 



thus rendering the subdivision exact. Direct readings may be 



obtained to 0"01 N.A. with this device. 



