LIGHT (AS. IFobt. Goth. liuhap, Bolt, Ger. Licht ; connected with Lat. lueere, to he bright, Gk. Xenc6s, frukog, bright, Ir. lorhe, lightning, OChureh Slay. lueho, beam of light, Skt. rife, to shine). The sensation of which one becomes conscious through the optic nerves. In general the cause of this sensation is the en trance into the eye of ethe•-waves whose wave numbers lie between certain limits: and there fore such unscientific expressions as 'light enters the eye.' light-waves.' etc.. are often used. From the standpoint of physics one is concerned with the problem of the production of these ether wa•es, their propagation and their entrance into the eye; after reaching the retina their further action becomes the study of physiology.
Any proper physical explanation of light must show that all the phenomena of ether-waves both in the pure ether and inside material bodies may be deduced from certain dynamical equa tions; and with our present knowledge of matter and the ether this can be done with a fair degree of satisfaction. There are, however, many
phenomena which do not require for the ex planation of their main features any exact of the properties of ether and matter, but which may be referred to certain general mechanical principles. Thus, the most important phenomena of interference and diffraction may be explained by the general prineiph•s of wave motion; and the most striking features of lenses, mirrors, and prisms may be shown to follow from the treatment of 'rays of light,' assuming the for reflection and refraction of rays.
That portion of the subject of light which may be explained by the use of the idea of rays is called 'ifekonetrical Opt ies:' that portion which uses the idea of wave-motion is called `Physical Opt ics;' and the complete treatment which explains the laws of rays and of waves and connects all the phenomena of dispersion, reflection, magnetie 1'1)U] tiOn, etc., by one set of dynamical equations is called the 'Dynamical Theory of Light.'