A valid formulation for the dynamical theory of light was first made by MacCullagh, who assumed that the aether was a new kind of material which opposed no resistance to compression or shear, but which resisted rotation. Such a material satisfies all the necessary conditions, but it suffers from the objection that there is no known kind of matter which has the property, and for this reason it was regarded with great suspicion and its importance was not appreciated till long afterwards. During the middle of the 19th century there were many attempts, especially by Lord Kelvin (then W. Thomson), to invent a sub stance which should satisfy all the necessary conditions, but they were mostly very artificial. The modern theory was finally formu lated by Clerk Maxwell, about 186o, who showed that electric oscillations must involve emitted waves which would have the same transverse character and would travel with the same veloc ity as light. He therefore identified light with electric waves, and gave the complete system of equations (then seen to be identical with MacCullagh's) which determine the behaviour of light. This theory has firmly held the field ever since, and Maxwell's name ranks as high as any among the contributors to optical theory. The completeness of his theory, and perhaps the familiarity that grows with the lapse of time, has overcome the objection that no known matter conforms to the same rules of vibration as the aether.
The Experimental Discoveries of the 19th Century.— During the period of these great theoretical investigations the experimental side had of course not been neglected, and many important discoveries had been made and instruments invented. Fraunhofer studied diffraction in a rather different way from Fresnel, and constructed gratings by winding wire between two fine screws and by ruling lines with a diamond on glass. With these he analysed the solar spectrum, and his work is thus the origin, both in subject matter and in method, of the modern science of spectroscopy. The invention of Nicol's prism (usually called the nicol) made it easy to produce polarized light, and this has ever since been the standard instrument in the study of polarization. Stokes began the study of ultra-violet light, render ing its effect visible by means of the property of fluorescence; it is now more usually investigated by photography. Faraday dis covered the theoretically very important phenomenon of mag netic gyration,—that, when a transparent substance is in a strong magnetic field, a beam of polarized light passing through it along the direction of the field has its direction of polarization rotated. This was the earliest connection discovered between light and electricity. Fizeau and Foucault developed methods of measuring the velocity of light accurately, and among other things showed that light really does go slower in water than in air, as is demanded by the wave theory (see FLUORESCENCE AND PHOSPHORESCENCE).
We must also mention the slow development of the theory of dispersion, i.e., the dependence of the refractive index on
the wave length. Fresnel and Cauchy propounded a theory which attributed it to the coarse-grainedness of the refracting medium, from which it followed that the refractive index could be ex panded in powers of the inverse square of the wave-length. This is often possible, but its inadequacy was seen when Leroux dis covered the phenomenon of "anomalous dispersion" (which is not really at all anomalous), that substances exist which refract the red more than the blue. A hint of the modern theory was given by Maxwell, but its real development is due to Sellmeier who showed that dispersion was an example of the general phenomenon of resonance. At the end of the I gth century Lorentz revised the whole of Maxwell's theory, introducing the idea of electrons, and in the course of his work improved Sell meier's dispersion formula.
During the closing decades of the 19th century an aspect of the wave theory which has had results of the most far reaching importance was much studied experimentally. Fresnel had suc cessfully treated the optical theory of moving media up to a point, but many difficulties remained with regard to the move ment of the earth as a whole. Stellar aberration suggested a fixed aether through which the earth moved, but then it seemed possible that for terrestrial experiments this motion ought to be detectable. On account of the great velocity of light none of these experiments were easy, but all gave negative results; particular mention may be made of the experiment of Michelson and Morley which was of such accuracy that, though it depended on the square of the ratio of the earth's velocity to that of light, it definitely established the negative. The mathematical theory was developed by Larmor and Lorentz to deal with this matter, and a very profound interpretation, given to it by Lorentz, led to the promulgation of the theory of relativity by Einstein (see RELATIVITY).
At the close' of the I gth century optical theory had attained a completeness and perfection which left hardly room for fur ther development, and it is with this classical theory that we shall here be concerned. During the loth century one most important branch has been added to it by the discovery of the interference of X-rays, but for the most part the centre of interest has shifted. The study of the conditions under which light is emitted has revealed fundamental difficulties in all our mechanical conceptions. This has led to the body of doctrine called the quantum theory (q.v.) which is antagonistic to the older classical system. Each in its own field covers a large number of phenomena, but the reconciliation is not yet complete, though it is already possible to rewrite the theory of dispersion in the new language. We shall not do this here, but shall treat it to gether with the rest of the subject by purely classical methods.