the rings gradually contracted; when the passage was from blue
to red, the rings expanded. This 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 ri