Optics is the science of the phenomena of light. This science is usually treated under two heads: (1) Physical Optics, which treats of the nature of Light (q.v., as also MAGNETISM and UNDULATORY THEORY), and explains the phenomena of Colour, Reflection, Refraction, Interference and its consequences, such as the colours of thin plates and films, Diffraction, Dispersion, the Spectrum, Polarisation and the properties of polarised light, for which see separate articles; and (2) Geometrical or Mathematical Optics. The leading idea in physical optics is to trace the progress of an undulatory or oscillatory disturbance in the Ether (q.v.); this disturbance, which may be termed a wave, has an advancing wave-front; the direction along which this wave-front advances through a given point is a geometrical conception, which it is convenient to make use of in diagrams, more convenient than it would be to draw a series of successive wave-fronts; this direction of propagation through any given point is called a ray; and geometrical optics traces, by mathematical reasoning, the course of a given set of 'rays' under specified conditions, particularly under those which have reference to Reflection and Refraction (q.v.). The part of geometrical optics which deals with reflection of light is often called Catoptrics (based on such laws as that the angle of reflection is equal to the angle of incidence); that which deals with refraction is called Dioptrics: and for an account of these, reference is made to articles REFLECTION and REFRACTION respectively.
Though the Greeks and their disciples the Arabs had made some progress in mathematical optics, their knowledge was confined to the law of reflection and its more immediate consequences. Euclid, Aristotle, Archimedes, Hero, and Ptolemy were acquainted with the fact that light is transmitted in straight lines; but, with the important exception of Aristotle and some of his followers, the ancient philosophers believed that rays proceeded from the eye to the object, instead of in the contrary direction. Ptolemy was well acquainted with atmospheric refraction. Alhacen (1070) and Vitellio the Pole (1260) were almost the only cultivators of this science during the middle ages, and their additions to it were unimportant. The lens, though known from early antiquity, was not applied as an aid to defective eyesight till after the time of Roger Bacon, Jansen, Metius, and Galileo separately invented the telescope about the beginning of the 17th century; and the last-mentioned philosopher by its means made various important astronomical discoveries. Kepler, a short time after, gave the true theory of the telescope, explained the method of finding the focal length of lenses, and applied it to find the magnifying power of the telescope, besides pointing out the mode of constructing an instrument better adapted for astronomical purposes than that of Galileo; he also made some useful experiments on the nature of colours, and showed that images formed on the retina of the eye are inverted, a fact previously discovered by Maurolycus of Messina. From this period the science of optics steadily advanced, and its treasury of facts received numerous additions through the labours of De Dominis, Snell (the discoverer of the law of refraction in 1621), Descartes, Fermat, Barrow, Mariotte, and Boyle. Up to the time of Newton it was generally believed that colour was produced by refraction, but that philosopher showed by a beautiful series of experiments that refraction only separates the colours already existing in white light. In his hands the theory and construction of the telescope underwent many valuable improvements, and in 1672 the description of his reflecting telescope was submitted to the Royal Society. Gregory had constructed an instrument on similar principles some years before. About the same time Grimaldi made his interesting series of experiments on the effects of diffraction, and noticed the remarkable fact of the interference of one pencil of light with the action of another. The theory of the rainbow, with an elegant analysis of the colours of thin plates, and the hypothesis concerning the nature and propagation of light, now known as the 'corpuscular' theory (see LIGHT), completed Newton's contributions to the science. The important services of the ingenious but eccentric Hooke cannot be easily stated in a brief abstract, as he discovered a little of everything, completed nothing, and occupied himself to a large extent in combating faulty points in the theories of his contemporaries. It must not, however, be forgotten that he has as much right as Huygens to the credit of originating the undulatory theory. The double refraction of Iceland spar was discovered (1669) by Bartholin, and fully explained in 1690 by Huygens, the propounder of the undulatory theory, who also aided the progress of mathematical optics to a considerable extent. The velocity of light was discovered by Römer (1675), and in 1720 the aberration of the fixed stars and its cause were made known by Bradley, who likewise determined with accuracy the amount of atmospheric refraction. Bouguer, Porterfield, Euler, and Lambert rendered essential service to physical optics; the same was done for the mathematical theory by Dollond (the inventor of the achromatic telescope), Clairaut, D'Alembert, Boscovich, &c.; while in later times the experiments of Delaval on the colours produced by reflection and refraction; the discussion of the phenomena arising from unusual reflection or refraction carried on by Vince, Wollaston, Biot, Monge, and others; the discovery of polarisation of light by Malus (1808), and its investigation by Brewster, Biot, and Seebeck; of depolarisation by Arago (1811), and of the optical properties as connected with the axes of crystals (1818) by Brewster; and the explanation of these and other optical phenomena in accordance with the undulatory hypothesis by Young—the discoverer of the Interference (q.v.) of rays—and Fresnel, went far to give optics a width of scope and a symmetry which are possessed by few other sciences. The development of the undulatory theory and of optical science generally has been carried on in the present century by Lloyd, Airy, Cauchy, Clerk-Maxwell, Hertz, and others; and for an account of the present state of the science reference may be made to Mr Thomas Preston's Theory of Light (1890), in addition to the works mentioned under LIGHT, and the articles LENSES, MICROSCOPE, TELESCOPE, &c.
Optimism (Lat. optimus, 'best'), the doctrine that the existing order of things, whatever may be
its seeming imperfections of detail, is nevertheless, as a whole, the most perfect or the best which could have been created, or which it is possible to conceive. Some of the advocates of optimism content themselves with maintaining the absolute position, that, although God was not by any means bound to create the most perfect order of things, yet the existing order is de facto the best; others contend that the perfection and wisdom of Almighty God necessarily require that His creation should be the most perfect which it is possible to conceive. The philosophical discussions of which this controversy is the development are as old as philosophy itself, and are dealt with in the article on the origin of Evil (q.v.). But the full development of the optimistic theory as a philosophical system was reserved for Leibnitz (q.v.), in his Theodicee, the main thesis of which is that, among all the systems which presented themselves to the infinite intelligence of God as possible, God selected and created in the existing universe the best and most perfect, physically as well as morally, regard being had to the universe as a whole. The Theodicee was designed to meet the sceptical theories of Bayle, and its theories were ridiculed in Voltaire's Candide. Modern discussion on this question usually assumes the form of assertion or denial of the opposite doctrine of Pessimism (q.v.).