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 the same equation of motion for the medium; but, owing to the differences in the surface-terms, they yield different conditions at the boundary of the medium, and consequently give rise to different theories of reflexion.

The assumptions involved in Maxwell's treatment of the magnetic rotation of light were such as might scarcely be justified in themselves; but since the discussion as a whole proceeded from sound dynamical principles, and its conclusions were in harmony with experimental results, it was fitted to lead to tho more perfect explanations which were afterwards devised by his successors. At the time of Maxwell's death, which happened in 1879, before he had completed his fortyninth year, much yet remained to be done both in this and in the other investigations with which his name is associated; and the energies of the next generation were largely spent in extending and refining that conception of electrical and optical phenomena whose origin is correctly indicated in its name of Maxwell's Theory.