is a beautiful experiment, and appears to have given Newton the most lively satisfaction.  When white light fell upon, the glasses, inasmuch as the colours were not superposed, a series of iris-coloured circles was obtained.  A magnified image of Newton’s rings is now before you, and, by employing in succession red, blue, and white light, we obtain all the effects observed by Newton.  You notice that in monochromatic light the rings run closer and closer together as they recede from the centre.  This is due to the fact that at a distance the film of air thickens more rapidly than near the centre.  When white light is employed, this closing up of the rings causes the various colours to be superposed, so that after a certain thickness they are blended together to white light, the rings then ceasing altogether.  It needs but a moment’s reflection to understand that the colours of thin plates, produced by white light, are never unmixed or monochromatic.

[Illustration:  Fig. 14]

Newton compared the tints obtained in this way with the tints of his soap-bubble, and he calculated the corresponding thickness.  How he did this may be thus made plain to you:  Suppose the water of the ocean to be absolutely smooth; it would then accurately represent the earth’s curved surface.  Let a perfectly horizontal plane touch the surface at any point.  Knowing the earth’s diameter, any engineer or mathematician in this room could tell you how far the sea’s surface will lie below this plane, at the distance of a yard, ten yards, a hundred yards, or a thousand yards from the point of contact of the plane and the sea.  It is common, indeed, in levelling operations, to allow for the curvature of the earth.  Newton’s calculation was precisely similar.  His plane glass was a tangent to his curved one.  From its refractive index and focal distance he determined the diameter of the sphere of which his curved glass formed a segment, he measured the distances of his rings from the place of contact, and he calculated the depth between the tangent plane and the curved surface, exactly as the engineer would calculate the distance between his tangent plane and the surface of the sea.  The wonder is, that, where such infinitesimal distances are involved, Newton, with the means at his disposal, could have worked with such marvellous exactitude.

To account for these rings was the greatest optical difficulty that Newton, ever encountered.  He quite appreciated the difficulty.  Over his eagle eye there was no film—­no vagueness in his conceptions.  At the very outset his theory was confronted by the question, Why, when a beam of light is incident on a transparent body, are some of the light-particles reflected and some transmitted?  Is it that there are two kinds of particles, the one specially fitted for transmission and the other for reflection?  This cannot be the reason; for, if we allow a beam of light which has been reflected from one piece of glass to fall upon another, it,