A luminous body is every body that is diffusive of its light. A diaphanous body is that through which the light passes. A shadowy (opaque) body, through which the light does not pass. A ray is the luminous line. / A direct ray is that which proceeds from the luminous body and does not break nor bend, like (Figure of) a straight ray. / A reflected ray is that which, proceeding from the luminous body and (striking) another body, finds an obstacle and returns backward, as it does in a mirror. (Figure of) a Mirror. A refracted, or truly broken, ray is that which passes from one medium to another rarer or denser and does not return backward but goes onward and breaks, like that which passes from air to water or from water to air. Example: (Figure with) air - water - refracted ray. Radial pyramid. (Figure) / A mirror is every polished body made either by art or by nature. The incident line is that which first multiplies and is produced from things upon the mirror. / The reflected line is that which, finding the mirror and not being able to pass, returns backward to the eye. (Figure with:) the visible thing - incident line - eye - reflected line - mirror. / The point of incidence is that where the incident line is reflected, and it is also called the point of reflection. (Figure with:) point of incidence and of reflection. The perpendicular upon the plane mirror is that which perpendicularly falls upon the point of reflection, as (Figure with: perpendicular) in the concave mirror (Figure), in the convex (Figure). / That surface which contains within itself the incident line, the reflected line and the perpendicular is called the surface of reflection. (Figure with:) Surface of reflection. / The cathetus of incidence is that line which is produced perpendicular from the thing seen upon the mirror, and that which is produced from the eye perpendicular upon the mirror is called the cathetus of reflection. / (Figure with:) thing seen - cathetus of incidence - incident line - mirror - eye - here the thing appears - cathetus of reflection; and at that point where the incident line joins with this cathetus, here the thing appears, for example at point a. / (Figure with:) Angle of incidence - thing seen - angle of incidence - eye - angle of reflection.