This defect may be corrected by a suitable combination of two or more
lenses, though not without again having similarly, as in the
correction of the first color, some faint remnants of color, the
aberrations of third or still higher order. But even the correction of
the third or still higher order may, if the angular aperture is very
wide, leave quite visible and disturbing remnants of color.
Another and not uncommon explanation of the cause of this unwelcome
color, though not so serious and damaging a charge to the maker of the
objectives, is its attribution to the so-called "secondary spectrum."
This error, like that previously mentioned, is certainly indicated by
the appearance of certain colors under certain conditions, but being,
as a rule, one of the least defects of even our best objectives in
most cases, it is probably not the true source of the disturbance.
The secondary spectrum is very commonly confounded with the chromatic
aberration of higher order. While the latter is produced by
imperfections in the form of the lens, the former is due to an
imperfection of the optical qualities of the material from which the
lens is constructed, the crown and flint glass.
A glass prism of any angle will project upon a white surface a
spectrum of any length, according to the arrangement of the light
source, the screen, and the prism.
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