DEFINING POWER. 



107 



low angle, but long working distance, though not suffi- 

 ciently corrected to work with deep eye-pieces. In the best 

 objectives of high angle the marginal rays are not cut off, 

 but corrected to the very edges by the application of a 

 wider back lens than usual ; the aperture is therefore larger, 

 but the objective possesses less penetration than one of 

 lower aperture, and the working distance has been materially 

 reduced. 



A small aperture objective may be constructed from 

 one of these more perfectly corrected lenses by the addition 

 of a diaphragm ; such an objective would bear deep eye- 

 pieces and possess a fair amount of penetration, though the 

 working distance may not be sensibly increased, owing to 

 the thickness of the lenses and their various curves not 

 being specially planned with this end in view. 



The defining power of an objective may be examined by 

 the employment of certain test objects obtainable from 

 Messrs. Norman or 

 Wheeler, until the 

 student has learned how 

 to prepare them for 

 himself. The pollen of 

 the Hollyhock (Althea 

 rasa), shown in Fig. 97, 

 is a useful test ; it must 

 be illuminated as an 

 opaque object, and with 

 a deep eye-piece (the 

 Huyghenian D), the 

 minute spines should 

 be readily and clearly 

 defined. 



A well cut wood-section, such as is shown in Fig. 98, is 

 also an excellent test of definition, the borders of each 



FIG. 97. 



