209 



The ArERTOMETER AND ITS USK, WITH A PeSCRITTION OF TwO 



Simple Forms of the Instrument. 



By H. F. Angus. 



(lit'ad Btrenih'r 2(V^, 19.M.) 



The Ap'.n-tometer, on acooiint of its high price and supposed 

 limited range of usefiihiess, is perhaps the h^st piece of apparatus 

 which the average worker would consider necessary, and few 

 possess sucli an instrument ; yet the apertometer is capable of 

 giving information which cannot be obtained in any other way — 

 information which materially assists in bringing out the utnu^t 

 capabilities of the optical combination used, and of eluciilating a 

 great number of those problems which are constantly presenting 

 themselves to the e;irnest student, without a clear understanding 

 of which the intelligent use of the microscoj>e is imjx^ssible. 



These two objections, however — viz., high price and limited 

 range of usefulness — need no longer deter those who prefer 

 scientitic methods of usiuii* a scientific instrument to the rouijh- 

 and-ready, and therefore inetiicient procedure usually in vogue, if 

 only immersion lenses are left out of consideration. Of aperto- 

 meters of limited range there are two patterns to which I wish 

 to draw your attention : one. whicli we will call the protractor 

 aj^ertometer, capable of measuring the aperture of any dry lens; 

 the other, the substage-scale apertometer, useful only for dry 

 objectives of medium aperture, its range of etticiency varying 

 somewhat witli the aplanatic apeitui'e of the condenser witli 

 which it is used. The former should not cost more than a quarter 

 of the price of the cheapest of those at present on tlie market, 



