instrument also a point situated vertically under the 

 object. Connect these points and with a protractor 

 measure the angle. 



A very accurate method and one which can be 

 carried out on all instruments is that suggested by 

 Prof. Abbe and for which the firm of Carl Zeiss supply 

 an apparatus which is called the apertometer, which 

 consists of a semi-circular disk of glass having at its 

 straight edge a beveled surface which reflects the light 

 through a perforated disk into the objective. Two 

 strips of brass which act as stops are placed on the arc 

 and thus indicate the limit of aperture, which, as well 

 as the corresponding numerical aperture, can be read 

 off on the scale. 



In the foregoing we have gone to some length in 

 stating the importance of angular and numerical aper- 

 ture and laid stress upon the influence which these 

 factors have upon the efficiency of the objective. We 

 shall now endeavor to explain what these attributes are 

 which determine the value of an objective. 



Resolving Power. Most important of all qualities 

 in an objective is the resolving power, which is the 

 power to show intricate structure and minute detail, it 

 being of course understood that the objective is properly 

 constructed so that defects shall not detract from this 

 quality. It is of course clear, that no matter how great 

 the numerical aperture may be, its effectiveness may 



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