91 



extreme on the table and beneath the 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 aperto- 

 meter, 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 can be read off on the 

 scale as well as the corresponding numerical 

 aperture. 



In the foregoing we have gone to some length in 

 stating the importance of angular and numerical 

 aperture 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. 



Resolving Power. First of all qualities in an 

 objective is the resolving power, which is the 

 ability 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 



