594 
Transactions of the Society. 
the how. The N.A. is then read off at the fixed index on the outer 
circle of figures hut one. 
It will be observed that when the calculator is once set, the N.A. 
of any number of objectives may be read off one after another by 
merely bringing their measured angles in a line with the movable 
index, by turning the milled head at the how. In brief, to find the N.A. 
of the first objective, three movements of the calculator are required, 
but for any subsequent objective only one movement of the calculator 
is necessary. Two significant figures are given, and a third can be 
estimated. By this means, half a dozen N.As. may be computed in 
the time required to take out one sine from the tables, let alone the 
multiplication sum.* 
* It may be of interest to know that Messrs. Zeiss supply a plain apertometer 
disc, without the internal lens and other fittings, for 25s. 
