1 10 



PRACTICAL MICROSCOPY. 



utmost perfection in resolution; but authorities differ on 

 this point. 



For higher powers a white petal of the Pelargonium may 

 serve as a test object, it is a moderately severe test for a 



FIG. 101. 



FIG. 102. 



J-inch ; while for higher powers still, the diatom Meridion 

 circulare will show directly whether the corrections have 

 been well executed. 



Insect scales are generally used for judging the defining 

 power of higher objectives, such as the scale of the Morpho 

 Menelaus shown in Fig. 102, and that of the Podura, 

 Fig. 103. 



The former exhibits lines in a longitudinal direction with 

 transverse markings, attributed to the corrugation of the 

 internal surfaces of the lining membrane, and which are 

 only to be noticed by the use of a good objective. 



The Podura scale forms an excellent test of definition ; 

 it was known for this purpose before the appearance of 



