RESOLVING POWER. 



117 



stated that it depends entirely upon large aperture, com- 

 bined, of course, with accuracy of the corrections for 

 sphericity and chromatism. The several pieces of appa- 

 ratus mentioned in the previous chapter are often essential 

 with objectives of wide 

 aperture, for it is ob- 

 vious that the advan- 

 tages would be lost if 

 sufficiently oblique rays 

 did not enter into the 

 formation of the image. 

 The Ross - Zentmayer 

 stand, used with a i-inch 

 or 2-inch objective as a 

 condenser, the radial 

 substage condenser of 

 Messrs. Swift and Son, 

 or the oil - immersion 

 condenser of Messrs. Powell and Lealand, are all capable 

 of producing light-rays of sufficient obliquity. 



Diatom frustules, as a rule, furnish tests for the resolving 

 property of medium and high-power objectives, to which 

 may be added the insect scales already shown ; but these 

 natural tests are all of very variable quality. Gyrosigma 

 formosum and G. angulatum, shown by Figs. 109 and no 

 respectively, are tests of the resolving property of the 

 \ and |-inch objectives, both of which have been engraved 

 from photographs taken by the late Dr. Redmayne, of 

 Bolton. 



Navicula rhomboides, shown in Fig. in, is used as a test 

 for the -|th objective and higher powers. It is rather a 

 difficult diatom to resolve properly without accessories, but 

 with the Powell and Lealand oil-immersion condenser it 

 may be managed without much trouble. 



FIG. 1 08. 



