MICROSCOPIC DRAWING AND ENGRAVING. 



I am informed by Mr. De La Eue, that bands engraved upon 

 other plates, were observed and computed by himself, Mr. Lister, 

 and Mr. Nobert, and the results now before me, are in such 

 accordance as to leave no* doubt of their general accuracy, the 

 discrepancy being so trifling as to be explained by the small errors 

 inevitable in such observations. 



It will be evident that microscopes, having different degrees 

 of power and efficiency, would be necessary to render the lines 

 composing the successive bands of such a series distinctly visible ; 

 to determine what power would be required for each band, it is 

 not at all necessary to have recourse to any microscopic observa- 

 tions; the question simply is, what is the degree of closeness 

 of the lines, that the naked eye can barely distinguish as 

 separate ; this will, of course, be somewhat different for different 

 eyes. 



29. The use of these ^test-plates in determining the power and 

 efficiency of microscopes, will be easily understood ; instruments 

 of low powers, such, for example, as from 100 to 200, will only 

 make the wider bands, such as A B and c, fig. 24, distinctly 

 visible, the closer ones, E F G, will be barely visible as dark bands, 

 but the lines composing them will not be seen, and the closest of 

 the series, n I K, will not be seen at all. In proportion as the 

 power and efficiency of the microscope is increased, more and 

 more of the bands will be visible as distinct series of lines. 



Mr. Nobert supplies test-plates, engraved with bands of diffe- 

 rent degrees of closeness, according to the power of the instruments 

 to which they are to be applied. 



30. In the Report of the Juries of the Great Exhibition of 1851, 

 page 268, it is stated, that to see the bands of a test-plate of 10 

 bands, such as that described above, a linear magnifying power of 

 100 is necessary for the wider bands, such as I and II, but that to 

 distinguish those of the closest band, such as x, a magnifying 

 power of 2000 is necessary. 



I think it is apparent that this statement is erroneous, being 

 evidently incompatible with the relative closeness of the lines of 

 the several bands. Thus, for example, while there are 11265 

 lines of the first band to an inch, there are 49910 lines of the 

 tenth band to an inch. Those of the latter are, therefore, only 

 4 times closer than those of the former ; and it is evident, that 

 if these bands be viewed with two microscopes, one having a mag- 

 nifying power 4 1 times greater than that of the other, with 

 proportional defining and illuminating powers, the lines com- 

 posing them will appear equally separated ; and since it is 

 admitted in the report, that a power of 100 will render the lines of 

 the first band visible, as it evidently will do, it will follow that 

 70 



