577 LIGHT Subject ive and objective mean ings of the word Light. SOUND may bo 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 frequently use 1 in the objective as in the subjective sense. Thus, as Sound may be defined in terms of the motion of 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 vne 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 arrange 1 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 Reflexions on the Mechanism of Propagation of Li"ht. Division of the Subject into Geometrical and Physical Optics. GEOMETRICAL OPTICS. Rectilinear Propagation of Light in Homogeneous Media, Shadows. Camera Obscura, &c. Intensity of Illumination as depending on the Distance of the Source and the Obliquity of the Rays. Brightness and Intrinsic Brightness. Photometry. Velocity of Light. Behaviour of Light at the Common Surface of Two Homogeneous Media. Reflexion. Plane, Spherical, and Cylindrical Mirrors. Real and Virtual Images. Single Refraction. Composite Nature of White Light. Refrac tive Index. Dispersion. Prisms; Fraunhofer s Lines. Irrationality at Dispersion. Achromatism. Lenses. Telescope, Microscope 1 are Spectrum. Refraction by Cylinder. Rainbow. Refraction in a Non-homogeneous Medium. Hamilton s Charac teristic 1< unction. Mirage. Absorption, Abnormal Dispersion, Fluorescence, Phosphorescence. PHYSICAL Orncs. UXDULATOHY THEORY. Nature and Propagation of Waves. Hivygens s Principle. Explanation of Reflexion 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 FT? PI" LoM ?fSeminndulation. Newton s Rings. Colours of flnii Plates and of Grooved Surfaces. Relation between Wave-Length and Refractive Index. Double Refraction. Wave-Surface in Iceland Spar I ;,,JJ amation - Tra n8verse Vibrations. Nature of Un polarized Plane, Circularly, and Elliptically Polarized Light. Nicol s 1 rism. Depolarization by Doubly-Refracting Plate. Fresnel s Khomb. Doppler s Principle. Measurement of the Relative Velocity of Luminous Source and Spectator. Under OPTICS (GEOMETRICAL, PHYSICAL, and PHYSIO LOGICAL) further developments will be given; and the connexion between light and radiant heat will be discussed under RADIATION. EARLY HISTORY OF THE SUBJECT. It is to sight that The we are mimly indebted for our knowledge of external ancients- things. All our other senses together, except under very knowled ( special conditions, do not furnish us with a tithe of the f oi>tics information we gain by a single glance. And sight is also that one of our senses which we are able most effectively and extensively to aid by the help of proper apparatus not merely (as by spectacles, invented circa 1300) for the cure of natural defects, but (as by the telescope and microscope) for the examination of bodies either too distant or too minute to be studied by the unassisted eye. It is very remarkable, under these circumstances, to find Light how slowly men have reached some even of the simplest llloves iu facts of optics. We can easily understand how constant * trai ht experience must have forced on them the conviction that light usually moves in straight lines, i.e., that we see an object in the direction in which it really lies. But how they could have believed for ages that objects are rendered visible by something projected from the eye itself so that the organ of sight was supposed to be analogous to the teutaculaof insects, and sight itself a mere species of touch is most puzzling. They seem not till about 350 B.C. to have even raised-the question If this is how we see, why cannot we see in the dark? or, more simply, What is darkness ] The former of these questions seems to have been first put by Aristotle. The nature and laws of reflexion were, of course, forced Re- ou the ancients by the images seen in still water ; and the flexion, geometers of the Platonic school were well acquainted with these laws. To Hero of Alexandria we owe the important deduction from them that the course of a reflected ray is the shortest possible. The general nature of refraction also was known, with Re- some of its special applications, such as, for instance, to fraction, burning-glasses and to magnifiers. These were probably efther spherical glass shells filled with water (Pliny, H.N xxxvi. 67 [25]; Lack, De Ira Dti, c. 10) or balls of rock crystal (Pliny, xxxvii. 10). In the first century of our era Cleomedes pointed out how a coin at the bottom of an empty cup, where the eye cannot see it, can be made visible by filling the cup with water ; and he showed that, in a similar way, the air may render the sun visible to us while it is still under the horizon. Shortly after this date Ptolemy (the celebrated astronomer) published his great work on Optics. He treats of_ vision, reflexion, the theory of plane and concave mirrors, and refraction. He measured, with considerable accuracy, the angles of incidence end refraction, for rays passing from air into water and into glass, and from water into glass; it was not, however, till more than fifteen hundred years had passed that the true relation between these angles was discovered. In addition to what has just been mentioned, the ancients knowledge of optics was limited to a very superficial acquaintance with some of the properties of rainbows, halos, mirage, cvc. But it was fragmentary in the extreme though it far surpassed in amount as well as in accuracy their knowledge of the other branches of physical science. It is not easy to understand the ideas of the ancients Colour about colour. That it is a property of a body just as its density, its hardness, or its smell is a property was probably held by them. But they also imagined that a body could communicate its colour to light; thus, for instance, the clouds were, by some of them, supposed XIY. - 73
