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Dr. Young has with equal success applied the principle of interferences to the explanation of the coloured rings, both reflected and transmitted, of thin plates. When such a plate is seen by reflexion, the light coming from the first surface to the eye interferes with that from the second; this interference either does or does not produce the sensation of light, according as the different distances that the rays have to pass over, place them in similar or opposite phases of their undulations; but then, at the point where the thickness is nothing, this difference is nothing, and consequently one would expect to see a bright spot instead of a dark one. To get over this difficulty Dr. Young introduces a new principle, namely, that the reflexion within the plate makes the rays lose an interval 12l, exactly equal to half the length of a wave. By means of this modification, the rays reflected from the two surfaces at the point where the thickness is nothing, acquire opposite dispositions, and therefore produce together no sensation of light in the eye; then in the surrounding places, the law of the periods of the undulations gives that of the succession of bright and dark rings: this law, thus modified, agrees with the measurements of the coloured rings observed in the case of perpendicular incidence; but for oblique incidences it is not quite consistent with Newton's statement. Is it possible that the laws which Newton established upon experiments may be inexact, or must we introduce in the case of oblique waves some modification depending on their impact on the surfaces? This point is yet to be decided.
We have hitherto considered only the rings observed by reflected light; the others are formed, according to the undulation system, by the interference of waves transmitted directly, with those which, being reflected at first at the second surface of the thin plate, are again reflected on returning to the first, and are thus sent to the eye at which they arrive without any farther modification. In this case the point where the surfaces touch should give a bright spot, as we find by experience that it does, so that here we have no additional principle to introduce as in reflexion; but this is quite necessary in many other cases.
According to this system, the thicknesses at which the rings are formed indicate the length of the oscillations in any substance. Now for one given mode of vibration of the luminous body, the
