MICROSCOPIC DRAWING AND ENGRAVING. 
. . • ;Av_. 
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, hut striking 
differences are found between scale and 
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. 
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