CATHETUS, in Geometry, a line or radius falling perpendicularly on another line or surface. Thus the catheti of a right-angled triangle are the two sides that include the right angle.
CATHETUS of Incidence, a right line drawn from a point of the object, perpendicular to the reflecting line.
CATHETUS of Reflection, or of the Eye, a right line drawn from the eye, perpendicular to the reflecting plane.
CATHETUS of Obliquation, a right line drawn perpendicular to the speculum, in the point of incidence or reflection.