used. They are made in the same powers as indicated 

 above and are superior to the achromatic lenses because 

 they have a more perfect chromatic and spherical cor- 

 rection. 



OPTICAL QUALITIES OF OBJECTIVES 



Numerical Aperture. N. A.=n. sin u. This 

 term was introduced by Abbe. n. is the refractive index 

 of the medium between the object and the front lens of 

 the objective (air in case of dry objectives and water or 

 oil in case of immerson objectives), and u is half the 

 angular aperture. 



Several important qualities of the objective depend 

 upon the numerical aperture. 



(a) RESOLVING POWER. This is directly proportion- 

 al to the numerical aperture and represents the ability 

 of the objective to show detail in the image of the object. 

 The higher the numerical aperture, the greater the re- 

 solving power, and the finer the detail we may expect to 

 see in the image. 



(b) DEPTH OF SHARPNESS, OR PENETRATION. This 

 is the power of an objective to show sharply objects 

 lying in different planes, one above another, without 

 the necessity of focusing up and down. The depth of 

 sharpness is in inverse ratio to the numerical aperture. 

 Therefore the lens of low numerical aperture has little 

 resolving power and great penetration. The lens of high 

 numerical aperture has great resolving power and little 

 penetration, unless it be used with a narrow cone of 

 light which practically makes it a lens of low aperture 

 with the qualities of such a lens. 



(c) ILLUMINATING POWER. The brilliancy of the 

 objective increases with the square of the numerical 

 aperture of the objective. An objective of .40 N. A. 



25 



