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 can be sensible only at very minute distances, and which are therefore quite analogous to those which are exerted in chemical affinities. It follows, that when particles of light are at a sensible distance from a refracting body, the effect they experience from it is quite inappreciable, so that their natural rectilinear direction is not altered; they begin to deviate from this direction only at the moment when they are in the immediate vicinity of the refracting surface, and the action takes place only for an infinitely short period of time; for as soon as the particles have penetrated within the surface to a distance ever so small, the forces exerted on them by the molecules of the medium become sensibly equal in all directions, so that the path of the light becomes again a straight line, though different from the preceding. It is therefore clear that the curved portion of the path being infinitely small, it must appear to consist, on the whole, of two straight lines forming an angle, which, in fact, is quite conformable to experience. But for the very reason that the curve is not perceptible, it is useless to seek, from experiment, any notions of its form that might lead to a knowledge of the laws which produce it, as observations on the orbits of the planets have led to a knowledge of the laws of gravitation. We must therefore have recourse to some other characters derived from experiment.

Isaac Newton has succeeded in the case of ordinary refraction, by considering each luminous molecule passing through a refracting surface, as acted on before, during, and after its passage, by attractive forces sensible only at very small distances, and emanating from all parts of the refracting medium. This definition specifies nothing as to the law of the attracting forces; it allows us only to calculate their resultant for any distance, and to suppose that they become evanescent when the distance is of sensible magnitude. Now these data are sufficient to calculate, not indeed the velocity of the molecules in their curvilinear motion, nor the nature of that motion, but only the relations of the final velocities and directions, which ensue, either in the medium or out of it, when the distance of the luminous molecules from the refracting surface is become so considerable that the trajectory is sensibly rectilinear, which will comprehend all distances that we can observe.

For extraordinary refraction, we have not the advantage of being able to define the origin of the molecular force, nor the manner in