in the second principal case.

As to double refraction, Fresnel made it depend on the unequal elasticity of the aether in different directions. He came to the conclusion that, for a given direction of the waves, there are two possible directions of vibration (§ 6), lying in the wave-front, at right angles to each other, and he determined the form of the wave-surface, both in uniaxal and in biaxal crystals.

Though objections may be urged against the dynamic part of Fresnel’s theory, he admirably succeeded in adapting it to the facts.

16. Electromagnetic Theory.—We here leave the historical order and pass on to Maxwell’s theory of light.

James Clerk Maxwell, who had set himself the task of mathematically working out Michael Faraday’s views, and who, both by doing so and by introducing many new ideas of his own, became the founder of the modern science of electricity,[16] recognized that, at every point of an electromagnetic field, the state of things can be defined by two vector quantities, the “electric force” E and the “magnetic force” H, the former of which is the force acting on unit of electricity and the latter that which acts on a magnetic pole of unit strength. In a non-conductor (dielectric) the force E produces a state that may be described as a displacement of electricity from its position of equilibrium. This state is represented by a vector D (“dielectric displacement”) whose magnitude is measured by the quantity of electricity reckoned per unit area which has traversed an element of surface perpendicular to D itself. Similarly, there is a vector quantity B (the “magnetic induction”) intimately connected with the magnetic force H. Changes of the dielectric displacement constitute an electric current measured by the rate of change of D, and represented in vector notation by

C = Ḋ

(11)

Periodic changes of D and B may be called “electric” and “magnetic vibrations.” Properly choosing the units, the axes of coordinates (in the first proposition also the positive direction of s and n), and denoting components of vectors by suitable indices, we can express in the following way the fundamental propositions of the theory.

(a) Let s be a closed line, σ a surface bounded by it, n the normal to σ. Then, for all bodies,

∫ Hsds = 1∫ Cn dσ,   ∫ Es ds = − 1 d∫ Bn dσ,
c cdt