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 other such number; whereas at points where the difference is l, 2l, 3l, or any other multiple of l, the undulating motions coincide, and assist each other, so that the appearance of light is produced.

This way of considering of the combination of luminous waves and the alternations of light and darkness which result from it has been called by Dr. Young, the Principle of Interferences.

The phænomenon of the alternations of light and darkness is certain; if, reasoning à priori, it appeared to be possible, only on the hypothesis of undulations, it would reduce the probability of that hypothesis to a certainty, and completely set aside the theory of emission. It does not, however, appear to offer that character of necessary truth which would be so valuable, whichever argument it favoured, because it would be decisive. One may, without violating any rule of logic, conceive equally the principle of interferences in the system of emissions, making the result which it expresses a condition of vision.

In fact, the phænomenon of the fringes does not prove that the rays of light really do affect each other under certain circumstances, it only shows that the eye does, or does not receive the sensation of light, when placed at a point where the rays coincide with those circumstances; it proves also that an unpolished surface placed at such a point, and seen from a distance, appears either bright or dark; now in the former case it is possible that vision may cease when the retina receives simultaneously rays which are at different epochs of their fits; and in the latter, when such rays arrive together at an unpolished surface, and are afterwards dispersed by radiation in all directions, it is clear, that having the same distance to pass over from each of the surfaces to the eye, they will have, on arriving at it, the same relative phases that they had when at the surface; if therefore they were then in opposite states, they will be so likewise in arriving at the retina, and thus there will be no vision produced. I do not pretend that this explanation is the true one, or even that it bears the character of necessity; it is both true and necessary if light be material, for it is but the statement of a phænomenon; but if only it implies no physical contradiction, that is quite sufficient to prevent the phænomenon from which it is derived from being decisive against the system of emission.