MICROSCOPIC DRAWING AND ENGRAVING. 



Fig. 21. 





25. ALTHOUGH these, and numerous other objects selected from 

 the minute parts of the animal kingdom, have been proposed, and 

 generally adopted, as microscopic tests ; they are subject to the 

 obvious objection, that, when considered as standards, they are 

 wanting in permanence and identity. Not only do the scales 

 taken from different individuals of the 

 same species differ in the fineness and 

 delicacy of their tracery, but striking- 

 differences are found between scale anct 

 scale, taken from the body of the same 

 individual insect. Thus, for example, the 

 scales shown in fig. 21, and that shown in 

 fig. 20, were taken from the same Podura, 

 yet fig. 21 requires a much more efficient 

 instrument to develop its tracery than 

 fig. 20. 



In fig. 22 is exhibited a scale of the 

 same Lepisma from which that represented 

 in fig. 19 was taken ; and which has been 

 drawn with the same magnifying power. 

 The tracings upon this are evidently much 

 ^^^^^^^^^^^^ more minute than those on fig. 19, and 

 are consequently shown with much less distinctness. It appears, 



Fig. 22. 



therefore, that these two scales, taken from the same individual 

 insect constitute different microscopic standards. 

 66 



