LIGHT, sound may be defined as any effect on the sense of hearing, and in the same way Light may be defined ' as any effect on the sense of sight. This is the purely subjective use of the terms. But both terms are quite as the air in the cavity of the external ear, mechanically affecting the tympanum, so Light may be defined by the mechanical effect produced upon the extension of the optic nerve which forms the sensitive surface of the retina.
In treating of Light it will be convenient to use the term in a sort of mixed sense, at least until we come to discuss the different theories which have been devised to account for the propagation of the agent which causes vision. Then we shall have to tile the term entirely in the objective sense. On the other hand, in Physiological Optics we are concerned chiefly with the subjective sense of the term.
The present article is intended to give a general sketch of the subject of Optics, so far as it can be treated by the help of elementary mathematics, but with sufficient detail to show the connexion of its various branches, and to enable the reader who desires further information on any point to judge for himself under what heading he will find it in this work. The subject is arrangel in the following order :- Early history of Optics.
Preliminary Statements with regard to Vision, Distinct Vision, the Colour-Sense, and the Duration of Visual Impressions.
Sources of Light.
General Rellexions on the Mechanism of Propagation of Light. Division of the Subject into Geometrical and Physical Optics.
RectilinearPropagation of Light in Homogeneous Media,Shadows. Camera Ohsenra, &c.
Intensity of Illumination as depending on the Distance of the Source and the Obliquity of the Rays. Brightness and Intrinsic It•ightness.
Velocity of Light.
Behaviour of Light at the Common Surface of Two Homogeneous Media.
Reflexion. Plane, Spherical, and Cylindrical Mirrors. Real mil Virtual Images.
Single Refraction. Composite Nature of White Light. Refractive Index. Dispersion. Prisms; Fraunhofer's Lines. Irrationality of Dispersion. Achromatism. Lenses. Telescope, Microscope. Pure Spectrum. Refraction by Cylinder. Rainbow.
Refraction in a Non-homogeneous Medium. Hamilton's Charm-t eristic Function. Mirage.
Absorption, AbnormdDispersion, Fluorescence, Phosphorescence.
Pu VSICAL OP rws. UNDULATORY THEORY.
Nature and Propagation of Waves. Ihwens's' Principle.
Explanation of Retlexion and Single Refraction. Disproof of the Corpuscular Theory.
Sketch of the History of the 'Undulatory Theory. Young's Discovery of Interference.
Interference Bands. Spectrum formed by Grating. Measure of Wave-Length. Loss of Seminndulation. Newton's Dings. Colours of Thin Plates and of Grooved Surfaces.
Relation between Wave-Length and Refractive Index. Double Refraction. Wave-Surface in Iceland Spar.
Polarization. Transverse Vibrations. Nature of Unpolarized Platte, Circularly, and Elliptically Polarized Light. Nicol's Doppler's Principle. Measurement of the Relative Velocity of Luninous Source and Spectator.
Under OPTICS (GEOMETRICAL, PHYSICAL, and PHYSIOLOGICAL) further developments will be given; and the connexion between light and radiant heat will be discussed under RADIATION.