USE OF TESTS. 
the glass upon which the lines a a ' and b b' are engraved, can be 
moved by a fine micrometer screw until the line b b' shall pass 
Fig. G, Fig. T. 
successively through the two extremities of the object, or the same 
purpose will be served, if, while the glass remains at rest, the 
stage which supports the object be similarly moved. 
The number of threads of the screw to an inch being known, the 
number of revolutions and parts of revolutions of the screw neces¬ 
sary to make the line pass from one extremity to another, will 
give the length of the object, and a like process will determine 
its breadth. 
In the application of such scales to microscopic measurements, 
various practical precautions and expedients are necessary, which 
will be fully explained in our Tract on the Microscope. 
14. Independently of being provided with means such as have 
been described above, for ascertaining the dimensions of objects, 
the advanced state of science renders it indispensable that the 
observer should possess means of testing the power of his instru¬ 
ment ; without such means, he can never be sure that the appear 
ance of the object, as presented by his microscope, corresponds 
with its real structure, or that important details of that structure 
may not escape his observation. A more striking example of 
this cannot be presented than one which was given by the late 
Dr. Goring, who showed that a particle of the dust taken from 
the wing of a certain species of butterfly, called the Morpho 
Menelaus, exhibited the seven different appearances shown in 
fig. 8; when viewed with the same microscope, the aperture of 
the object-glass and, consequently, the brightness of the image 
only being varied. It will be seen that details of structure are 
rendered apparent in G, where the aperture is greatest, which are 
very imperfectly shown in F, and not at all in those in which 
the aperture was still more limited. 
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