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OPTICAL ERRORS AND HUMAN MISTAKES.[1]
[Footnote 1: Read before the American Association, Buffalo,
August, 1886.]
By ERNST GUNDLACH.
I wish to call attention to a few mistakes that are quite commonly
made by microscopists and writers in stating the result of their
optical tests of microscope objectives.
If the image of an object as seen in the microscope appears to be
unusually distorted and indistinct toward the edge of the field, and
satisfactory definition is limited to a small portion of the center,
the cause is often attributed to the spherical aberration of the
objective, while really this phenomenon has nothing to do with that
optical defect of the objective, if any exists, but is caused by a
lack of optical symmetry. If a perfectly symmetrical microscope
objective could be constructed, then, with any good eye-piece, it
would make no difference to the definition of the object were it
placed either in the center or at the edge of the field, even if the
objective had considerable spherical aberration. But, unfortunately,
our most symmetrical objectives, the low powers, leave much to be
desired in this respect, while our wide angle, high powers are very
far from symmetrical perfection.
There are two causes of this defect in the latter objectives, one
being the extreme wideness of their angular apertures, and the other
the great difference in the distances of the object and the image from
the optical center of the objectives.
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