will give an image four times as brilliant as one of .20 

 N. A., provided the magnification is the same and the 

 full cone of the illumination is used in both cases. 



Magnifying Power. The magnifying power of an 

 objective is in inverse ratio to its focal distance. An 

 objective of 2mm focal distance will give, with the same 

 ocular, a magnification eight times greater than one of 

 1 6mm focal distance. Numerical aperture and mag- 

 nifying power are of little advantage if the definition is 

 not good. 



Definition. The definition of an objective is char- 

 acterized by the cleanness and sharpness of the outlines 

 of the image. 



Definition depends upon the corrections for chromatic 

 and spherical aberrations, and the workmanship; the 

 centering of the lenses, etc. 



CHROMATIC ABERRATION is due to the fact that a ray 

 of white light passing from one medium to another of 

 different refractive index at any angle other than 90 

 to the surface between them is refracted and dispersed 

 into its component colors. 



SPHERICAL ABERRATION is due to the fact that a 

 spherical surface cannot bring a beam of light which 

 passes through its vertex to the same focus as that of 

 a beam of light passing through any other zone. 



Both aberrations are corrected by the use of different 

 kinds of glass (crown and flint) combined as double and 

 triple lenses in the objective. Neither can be corrected 

 absolutely for all colors in an achromatic objective. 

 Apochromatic objectives approach the ideally corrected 

 objective almost to perfection. 



An objective can be tested for chromatic correction 

 by using a narrow cone of oblique light and a coarse 

 26 



