Another mistake is often made in regard to the cause of certain
prismatic colors that are sometimes, in a striking degree, produced by
otherwise good objectives. According to the nature of these colors,
whether yellow or blue, green or indigo, they are generally regarded
as evidences of either chromatic over or under correction of the
objective. Of course the presence of either of these defects is
certainly and correctly indicated by the appearance of one or the
other of the colors, under certain circumstances; but the simple
visibility of prismatic color is by no means a reliable indication of
over or under correction of color, and, indeed, to the honor of our
opticians, it may be stated that very few objectives are made that
cannot justly be called achromatic in the general sense of the term.
By far the most common causes of prismatic color, in otherwise
carefully constructed objectives, are the so-called chromatic
aberrations of second or higher order. Every achromatic lens which is,
as it should be, at its best at about two-thirds of its aperture, is
inside of this ring or zone, toward the center slightly under and
outside, toward the edge, slightly over corrected. This defect is the
greater, the less the difference of the dispersive powers of the two
glasses used in the construction of the lens, for a given proportion
of their refractive indexes, and therefore the degree of visibility of
the colors of the aberrations of the second order depends greatly on
the nature of the glass employed in the construction of the lens.
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