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. 7. 



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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