INCIDENCE

, or line of Incidence, in Mechanics, implies the direction or inclination in which one body strikes or acts on another.——In the incursions of two moving bodies, their Incidence is said to be Direct or Oblique, as the directions of their motion make a straight line, or an angle at the point of Impact.

Angle of Incidence, by some writers, denotes the angle comprehended between the line of Incidence, and a perpendicular to the body acted on at the point of Incidence. Thus, suppose AB an Incident line, and BF a perpendicular to the plane CB at the incident point B; then ABF is the angle of Incidence, or of inclination.

But, according to Dr. Barrow, and some other writers, the Angle of Incidence is the complement of the former, or the angle made between the incident line, and the plane acted on, or a tangent at the point of Incidence; as the angle ABC.

It is demonstrated by optical writers, 1st, That the Angle of Incidence, of the rays of light, is always equal to the angle of reflection; and that they lie in the same plane. And the same is proved by the writers on Mechanics, concer^.ning the reflection of elastic bodies. That is, the [angle]ABF = the [angle]FBD, or the , That the sines of the Angles of Incidence and refraction are to each other, either accurately, or very nearly, in a given or constant ratio.—3dly, That from air into glass, the sine of the Angle of Incidence, is to the sine of the angle of refraction, as 300 to 193, or nearly as 14 to 9: and, on the other hand, that out of glass into air, the sign of the Angle of Incidence, is to the sine of the angle of refraction, as 193 to 300, or as 9 to 14 nearly.

Incidence of Eclipse. See Eclipse and IMMER- SION.

Axis of Incidence, is the line FB perpendicular t<*> the reflecting plane at the point of Incidence B.

Cathetus of Incidence. See Cathetus, and REFLECTION.

Line of Incidence, in Catoptrics, denotes a right line, as AB, in which light is propagated from a radiant point A, to a point B, in the surface of a speculum. The same line is also called an Incident ray.

Line of Incidence, in Dioptrics, is a right line, as AB, in which light is propagated unrefracted, in the same medium, from the radiant point to the surface of the refracting body, CBE.

Point of Incidence, is the point B on the surface of the reflecting or refracting medium, on which the Incident ray falls.

Scruples of Incidence. See Scruples.

INCIDENT Ray, is the line or ray AB, falling on the surface of any body, at B.