DEFINING POWER. 



105 



working distances than dry lenses, so that it is often pos- 

 sible to use an immersion ^-inch where the covering-glass 

 is too thick for a dry Jth. 



The author would like to see objectives catalogued by 

 the makers in somewhat the following manner ; the oculars 

 too might be included : 



DEFINING POWER. This property is of the first import- 

 ance in objectives, and has been described by Dr. Goring 

 to mean " nothing more than a destitution of both kinds of 



FIG. 94, 



aberration." A well-corrected objective focusses the indi- 

 vidual rays of a pencil of light, both from the centre and 

 periphery, to the same plane as shown at F in Fig. 94 ; but, 

 nevertheless, a more or less distinct image is produced for 



