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NATURE] components, and in 1904 Mills measured their velocities. The Kerr effect, discovered in 1877, and the Zeeman effect (1896) widened the field of research, which, from its intimate connexion with the nature of light and electromagnetics, has resulted in discoveries of the greatest importance.

§ 14. Optical Instruments.—Important developments have been made in the construction and applications of optical instruments. To these three factors have contributed. The mathematician has quantitatively analysed the phenomena observed by the physicist, and has inductively shown what results are to be expected from certain optical systems. A consequence of this was the detailed study, and also the preparation, of glasses of diverse properties; to this the chemist largely contributed, and the manufacture of the so-called optical glass (see ) is possibly the most scientific department of glass manufacture. The mathematical investigations of lenses owe much to Gauss, Helmholtz and others, but far more to Abbe, who introduced the method of studying the aberrations separately, and applied his results with conspicuous skill to the construction of optical systems. The development of Abbe’s methods constitutes the main subject of research of the present-day optician, and has brought about the production of telescopes, microscopes, photographic lenses and other optical apparatus to an unprecedented pitch of excellence. Great improvements have been effected in the stereoscope. Binocular instruments with enhanced stereoscopic vision, an effect achieved by increasing the distance between the object glasses, have been introduced. In the study of diffraction phenomena, which led to the technical preparation of gratings, the early attempts of Fraunhofer, Nobert and Lewis Morris Rutherfurd, were followed by H. A. Rowland’s ruling of plane and concave gratings which revolutionized spectroscopic research, and, in 1898, by Michelson’s invention of the echelon grating. Of great importance are interferometers, which permit extremely accurate determinations of refractive indices and wave-lengths, and Michelson, from his classical evaluation of the standard metre in terms of the wave-lengths of certain of the cadmium rays, has suggested the adoption of the wave-length of one such ray as a standard with which national standards of length should be compared. Polarization phenomena, and particularly the rotation of the plane of polarization by such substances as sugar solutions, have led to the invention and improvements of polarimeters. The polarized light employed in such instruments is invariably obtained by transmission through a fixed Nicol prism—the polarizer—and the deviation is measured by the rotation of a second Nicol—the analyser. The early forms, which were termed “light and shade” polarimeters, have been generally replaced by “half-shade” instruments. Mention may also be made of the microscopic examination of objects in polarized light, the importance of which as a method of crystallographic and petrological research was suggested by Nicol, developed by Sorby and greatly expanded by Zirkel, Rosenbusch and others.

.—There are numerous text-books which give elementary expositions of light and optical phenomena. More advanced works, which deal with the subject experimentally and mathematically, are A. B. Bassett, Treatise on Physical Optics (1892); Thomas Preston, Theory of Light, 2nd ed. by C. F. Joly (1901); R. W. Wood, Physical Optics (1905), which contains expositions on the electromagnetic theory, and treats “dispersion” in great detail. Treatises more particularly theoretical are James Walker, Analytical Theory of Light (1904); A. Schuster, Theory of Optics (1904); P. Drude, Theory of Optics, Eng. trans. by C. R. Mann and R. A. Millikan (1902). General treatises of exceptional merit are A. Winkelmann, Handbuch der Physik, vol. vi. “Optik” (1904); and E. Mascart, Traité d’optique (1889–1893); M. E. Verdet, Leçons d’optique physique (1869, 1872) is also a valuable work. Geometrical optics is treated in R. S. Heath, Geometrical Optics (2nd ed., 1898); H. A. Herman, Treatise on Geometrical Optics (1900). Applied optics, particularly with regard to the theory of optical instruments, is treated in H. D. Taylor, A System of Applied Optics (1906); E. T. Whittaker, The Theory of Optical Instruments (1907); in the publications of the scientific staff of the Zeiss works at Jena: Die Theorie der optischen Instrumente, vol. i. “Die Bilderzeugung in optischen Instrumenten” (1904); in S. Czapski, Theorie der optischen Instrumente, 2nd ed. by O. Eppenstein (1904); and in A. Steinheil and E. Voit, Handbuch der angewandten Optik (1901). The mathematical theory of general optics receives historical and modern treatment in the Encyklopädie der mathematischen Wissenschaften (Leipzig). Meteorological optics is fully treated in J. Pernter, Meteorologische Optik; and physiological optics in H. v Helmholtz, Handbuch der physiologischen Optik (1896) and in A. Koenig, Gesammelte Abhandlungen zur physiologischen Optik (1903).

The history of the subject may be studied in J. C. Poggendorff, Geschichte der Physik (1879); F. Rosenberger, Die Geschichte der Physik (1882–1890); E. Gerland and F. Traumüller, Geschichte der physikalischen Experimentierkunst (1899); reference may also be made to Joseph Priestley, History and Present State of Discoveries relating to Vision, Light and Colours (1772), German translation by G. S. Klügel (Leipzig, 1775). Original memoirs are available in many cases in their author’s “collected works,” e.g. Huygens, Young, Fresnel, Hamilton, Cauchy, Rowland, Clerk Maxwell, Stokes (and also his Burnett Lectures on Light), Kelvin (and also his Baltimore Lectures, 1904) and Lord Rayleigh. Newton’s Opticks forms volumes 96 and 97 of Ostwald’s Klassiker; Huygens’ Über d. Licht (1678), vol. 20, and Kepler’s Dioptrice (1611), vol. 144 of the same series.

Contemporary progress is reported in current scientific journals, e.g. the Transactions and Proceedings of the Royal Society, and of the Physical Society (London), the Philosophical Magazine (London), the Physical Review (New York, 1893 seq.) and in the British Association Reports; in the Annales de chimie et de physique and Journal de physique (Paris); and in the Physikalische Zeitschrift (Leipzig) and the Annalen der Physik und Chemie (since 1900: Annalen der Physik) (Leipzig).

1. Newton’s Corpuscular Theory.—Until the beginning of the 19th century physicists were divided between two different views concerning the nature of optical phenomena. According to the one, luminous bodies emit extremely small corpuscles which can freely pass through transparent substances and produce the sensation of light by their impact against the retina. This emission or corpuscular theory of light was supported by the authority of Isaac Newton, and, though it has been entirely superseded by its rival, the wave-theory, it remains of considerable historical interest.

2. Explanation of Reflection and Refraction.—Newton supposed the light-corpuscles to be subjected to attractive and repulsive forces exerted at very small distances by the particles of matter. In the interior of a homogeneous body a corpuscle moves in a straight line as it is equally acted on from all sides, but it changes its course at the boundary of two bodies, because, in a thin layer near the surface there is a resultant force in the direction of the normal. In modern language we may say that a corpuscle has at every point a definite potential energy, the value of which is constant throughout the interior of a homogeneous body, and is even equal in all bodies of the same kind, but changes from one substance to another. If, originally, while moving in air, the corpuscles had a definite velocity v0, their velocity v in the interior of any other substance is quite determinate. It is given by the equation mv2 − mv02＝A, in which m denotes the mass of a corpuscle, and A the excess of its potential energy in air over that in the substance considered.

A ray of light falling on the surface of separation of two bodies is reflected according to the well-known simple law, if the corpuscles are acted on by a sufficiently large force directed towards the first medium. On the contrary, whenever the field of force near the surface is such that the corpuscles can penetrate into the interior of the second body, the ray is refracted. In this case the law of Snellius can be deduced from the consideration that the projection w of the velocity on the surface of separation is not altered, either in direction or in magnitude. This obviously requires that the plane passing through the incident and the refracted rays be normal to the surface, and that, if 1 and 2 are the angles of incidence and of refraction, v1 and v2 the velocities of light in the two media,

The ratio is constant, because, as has already been observed, v1 and v2 have definite values.

As to the unequal refrangibility of differently coloured light, Newton accounted for it by imagining different kinds of corpuscles. He further carefully examined the phenomenon of total reflection, and described an interesting experiment connected with it. If one of the faces of a glass prism receives on the inside a beam of light of such obliquity that it is totally reflected under ordinary circumstances,