noeert’s test plates. 
26. The erroneous estimates of the relative efficiency and power 
of different microscopic instruments which would result from the 
use of such test-objects, are obvious. A microscopist in London, 
observing the tracery of the scale of a Podura, and another at 
New York observing another scale of the same insect, the former 
failing to see its striae, which would be visible to the latter, it 
could not at all be safely inferred, that the instrument of the one 
was inferior in efficiency and power to that of the other ; and it 
might even happen, that the instrument which failed to show the 
striae in London, was, nevertheless, superior to that which ren¬ 
dered them distinctly visible in New York. The result of such a 
comparison would entirely depend upon the structure of the two 
scales adopted as tests, which might differ within very wide limits. 
Independently of this uncertainty attending the application of 
such tests, there is another not less serious objection to them; 
they hold out a temptation to microscope makers who supply them 
with the instruments they sell, to select such only as are most 
easily rendered visible ; and although it be true that this is an 
expedient to which the most respectable class of makers would not 
resort, it is nevertheless true that the inferior makers do so, and 
thereby do injustice to those who are above such practices. 
Natural objects, therefore, do not supply such permanent and 
unalterable tests for the microscope as the double stars, stellar 
clusters, and nebulae do for the telescope; and this circumstance 
has directed the attention of the higher class of artists, to the pro¬ 
duction of artificial test-objects which shall have determinate and 
certain qualities, and which, like manufactured articles, may be 
reproduced with such absolute identity as to supply standards of 
comparison that can be applied in different places, and at different 
times, to different instruments, so as to give results which will 
admit of comparison. 
27. The production of micrometer scales, by Mr. Froment, the 
divisions of which are separated by intervals so small as the 
25000th of an inch, has been already mentioned. 
Now the lines marking such divisions being in closer proximity 
than those of the tracings upon certain test-objects, it will be 
evident that artificial test-objects might be made by means similar 
to those by which such scales have been executed, and there can 
be little doubt that the great artistic skill which has succeeded in 
producing traces, separated by the small interval above named, 
could be pushed further, so as to produce striated surfaces, which 
would serve all the purposes of test-objects. 
Mr. Nobert, of Griefswall, in Prussia, has taken up this problem 
of test-objects, and, without attempting, as it would appear, to 
engrave micrometric scales, which would require intervals of some 
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