Emily Dickinson Lexicon

AC-CI-DENT'AL, a.

1. Happening by chance, or rather unexpectedly; casual; fortuitous; taking place not according to the usual course of things; opposed to that which is constant, regular, or intended; as, an accidental visit.
2. Non-essential; not necessarily belonging to; as songs are accidental to a play. Accidental colors, are those which depend upon the affections; of the eye, in distinction from those which belong to the light itself. – Encyc. Accidental pond, in perspective, is that point in the horizontal line, where the projections of two lines parallel to each other meet the perspective plane.