TESTS OF DEFINITION. 



Ill 



Pritchard's 'Microscopic Cabinet,' published in 1832; but 

 the true character of the markings was not then known. 

 This scale was then included amongst the " line tests," 

 whilst now, with objectives of wide aperture, it presents 



FIG. 103. 



FIG. 104. 



the appearance shown in the figure. Central light from 

 an achromatic condenser is the best for exhibiting this 

 scale, or it may be illuminated as an opaque object by 

 means of the lamp and bull's-eye condenser. 



Other insect scales are to be found in use as test-objects, 

 such as those of the Lepisma saccharina shown in Fig. 104, 

 and the battledoor scales of the Polyommatus argus de- 

 lineated by Fig. 105. 



These are considered much easier tests than those of the 

 M. Menelaus and Podura (Lepidocyrtus curvicollis\ but not 

 so variable in quality. Fig. 106 shows one of the ordinary 

 scales of the Morpho Menelaus, being, however, magnified to 

 but one-half the extent of Fig. 105. 



