附加灭点

Accidental \Ac`ci*den"tal\, a. [Cf. F. accidentel, earlier
accidental.]
1. Happening by chance, or unexpectedly; taking place not
according to the usual course of things; casual;
fortuitous; as, an accidental visit.

2. Nonessential; not necessary belonging; incidental; as, are
accidental to a play.

{Accidental chords} (Mus.), those which contain one or more
tones foreign to their proper harmony.

{Accidental colors} (Opt.), colors depending on the
hypersensibility of the retina of the eye for
complementary colors. They are purely subjective
sensations of color which often result from the
contemplation of actually colored bodies.

{Accidental point} (Persp.), the point in which a right line,
drawn from the eye, parallel to a given right line, cuts
the perspective plane; so called to distinguish it from
the principal point, or point of view, where a line drawn
from the eye perpendicular to the perspective plane meets
this plane.

{Accidental lights} (Paint.), secondary lights; effects of
light other than ordinary daylight, such as the rays of
the sun darting through a cloud, or between the leaves of
trees; the effect of moonlight, candlelight, or burning
bodies. --Fairholt.

Syn: Casual; fortuitous; contingent; occasional;
adventitious.

Usage: {Accidental}, {Incidental}, {Casual}, {Fortuitous},
{Contingent}. We speak of a thing as accidental when
it falls out as by chance, and not in the regular
course of things; as, an accidental meeting, an
accidental advantage, etc. We call a thing incidental
when it falls, as it were, into some regular course of
things, but is secondary, and forms no essential part
thereof; as, an incremental remark, an incidental
evil, an incidental benefit. We speak of a thing as
casual, when it falls out or happens, as it were, by
mere chance, without being prearranged or
premeditated; as, a casual remark or encounter; a
casual observer. An idea of the unimportant is
attached to what is casual. Fortuitous is applied to
what occurs without any known cause, and in opposition
to what has been foreseen; as, a fortuitous concourse
of atoms. We call a thing contingent when it is such
that, considered in itself, it may or may not happen,
but is dependent for its existence on something else;
as, the time of my coming will be contingent on
intelligence yet to be received.