MICROSCOPIC DRAWING AND ENGRAVING. 
insects; the striae and dots upon which could he seen with more 
or less distinctness, according to the excellence of the instrument. 
It was to these that the name test- 
objects was first applied. 
22. As the microscope has been 
improved in its power from year to 
year, these test-objects have in¬ 
creased in number ; new details of 
structure being developed by every 
increase of power and efficiency in 
the instrument. A certain list of 
such objects has been agreed upon 
by general consent, and prepared for 
sale by the makers, consisting of 
hairs, scales, and feathers of insects ; 
as, however, it is not my present 
purpose to enter into any explana¬ 
tion on the subject of microscopic 
tests, except so far as may be neces¬ 
sary to' elucidate one of the uses of 
microscopic engraving, it will be 
sufficient here to give a few examples of these test-objects. 
23. There is a little insect, vulgarly called the silver-fish, or the 
silver-lady , of which the proper entomological name is the Lepisma- 
Saccharina ; it is usually found in damp and mouldy cupboards, 
and in old wood-work, such as window-frames. The silvery lustre 
from which it takes its vulgar name, proceeds from a coating of 
scale-armour, with which its entire body is invested. These scales, 
when detached from the insect, and examined with a microscope, 
present a beautiful striated appearance ; their magnitude varies; 
one, whose length is the 114th, and width the 170th part of an inch, 
is shown in fig. 19, as it appears in a good microscope, magnified 
400 times in its linear, and therefore 160000 times in its super¬ 
ficial dimensions. 
The scale, as here shown, is divided along the middle of its 
breadth, by a sort of geometrical axis, on either side of which the 
structure is perfectly similar. A regular series of striated lines 
diverge from this axis, at an angle of about 45°, intersected by 
another series, very nearly parallel to the axis. 
The divergent strise are very slightly curved; the concavity 
being presented downwards, and the longitudiual ones ought to 
appear with a microscope to stand out in bold relief, like the 
ribs seen on certain shells ; they are more closely arranged as they 
approach the lower part of the scale, and become more prominent 
as they are more separated in proceeding -upwards. 
62 
