line of incidence

Webster's Dictionary (GCIDE)

1.A falling on or upon; an incident; an event; an occurrence. [Obs.] --Bp. Hall. [1913 Webster]

2.(Physics) The direction in which a body, or a ray of light or heat, falls on any surface. [1913 Webster]

In equal incidences there is a considerable inequality of refractions.— Sir I. Newton.[1913 Webster]

3.The rate or ratio at which something occurs; as, the incidence of murder in Los Angeles; the incidence of cancer in men over 50. [PJC]

Angle of incidence, the angle which a ray of light, or the line of incidence of a body, falling on any surface, makes with a perpendicular to that surface; also formerly, the complement of this angle.

Line of incidence, the line in the direction of which a surface is struck by a body, ray of light, and the like. [1913 Webster]