214 H. F. ANGUS 0^" THE APERTOMETER AND ITS USE. 



practically the same plane as the iris, a direct reading can be 

 taken, not only of the N.A. of the objective, but also of the 

 iris aperture; the reading in this case is 8 mm., consequently 

 a 12 mm. stop will be required. 



To take another instance, an objective |-in. of N.A. 0-20 

 combined with an Abbe chromatic condenser of N.A. 1*20 ^vith 

 the front lens removed, gives a field of 12 mm., and it will be 

 found that an 18 mm. stop gives the best possible result. 



Instances might be multiplied ; but I think that, although the 

 above does not pretend to be exhaustive, enough has been said to 

 show the utility of an apertometer for this work. In conclusion, 

 I may say that black paper discs carefully centred on discs of 

 glass cut to the size of the stop carrier are quite efficient, at any 

 rate for experimental work : a set of stops so determined could 

 afterwards be made in metal if desired, the various discs being 

 made interchangeable on a skeleton stop carrying a central pin. 



Journ. Qv.ekett Microscopical Club, Ser. 2,, Vol. VIII., No. 50, April 1902* 



