Light is that principle or substance which renders objects perceptible to our sense of seeing. It is a subject apparently but little understood, and upon which opinions are as much divided as any of the most abstruse subjects of philosophical inquiry. Some consider light as a fluid material; others regard it as a " vibration propagated from the luminous body through a subtile ethereal medium," etc. The limits and design of this work equally preclude the insertion of an extended article on the various hypothesis that have been promulgated on this interesting subject. See Optics.