OBJECTIVES AND OCULARS 7 



or, in other words, the resolving power of an objective can be 

 denned as the minimum distance apart two lines or spots may be 

 and yet appear as two distinct individuals. The resolving power 

 of an objective is dependent upon its light collecting and light 

 transmitting power; this in turn is governed by the numerical 

 aperture and by the particular wave length of light entering 

 the lens system. 



From the viewpoint of the physicist the resolving power of 



an objective can be expressed as equivalent to , where X is 



the wave length of light. This is based upon the assumption 

 that the illuminating cone of light completely fills the aperture 

 of the objective. From this formula, we find that, theoretically, 

 the limit of resolution will be attained when the magnification 

 of an objective reaches about 900. 



The chemist is not alone interested in the brightness of image 

 and the resolving power of an objective, but he is vitally con- 

 cerned with another property, namely, the ability of the objective 

 to make clear objects or structures in more than one plane. 

 This is known as its penetrating power. The penetrating power 

 of an objective has been shown to be inversely proportional to 

 the numerical aperture and to vary as the square of the equiv- 

 alent focus. 



Leaving out of consideration the numerical aperture, it is 

 found that the resolving power of an objective is inversely pro- 

 portional to the wave length of light. By employing light 

 rays of very short wave lengths we may thus obtain exceptional 

 resolution. 



In the above consideration it has been assumed that the illu- 

 minating cone of light completely fills the aperture of the objective. 

 Nelson 1 has shown that in practice with the older types of 

 objective we can rarely count upon more than three-fourths 

 of the available numerical aperture. More modern objectives 

 perform somewhat better. 



In comparing objectives as to their ability to render struc- 

 tures clear and distinct it is usual to do so by computing the 



1 J. Roy. Micro. Soc., 1893, 15-17. 



