It has been already stated that the Emission Theory assigned a greater velocity to light in glass and water than in air or stellar space; and that on this point it was at direct issue with the theory of undulation, which makes the velocity in air or stellar space greater than in glass or water.  By an experiment proposed by Arago, and executed with consummate skill by Foucault and Fizeau, this question was brought to a crucial test, and decided in favour of the theory of undulation.

In the present instance also the two theories are at variance.  Newton assumed that the action which produces the alternate bright and dark rings took place at a single surface; that is, the second surface of the film.  The undulatory theory affirms that the rings are caused by the interference of waves reflected from both surfaces.  This also has been demonstrated by experiment.  By a proper arrangement, as we shall afterwards learn, we may abolish reflection from one of the surfaces of the film, and when this is done the rings vanish altogether.

Rings of feeble intensity are also formed by transmitted light.  These are referred by the undulatory theory to the interference of waves which have passed directly through the film, with others which have suffered two reflections within the film, and are thus completely accounted for.

Sec. 10. The Diffraction of Light.

Newton’s espousal of the Emission Theory is said to have retarded scientific discovery.  It might, however, be questioned whether, in the long run, the errors of great men have not really their effect in rendering intellectual progress rhythmical, instead of permitting it to remain uniform, the ‘retardation’ in each case being the prelude to a more impetuous advance.  It is confusion and stagnation, rather than error, that we ought to avoid.  Thus, though the undulatory theory was held back for a time, it gathered strength in the interval, and its development within the last half century has been so rapid and triumphant as to leave no rival in the field.  We have now to turn to the investigation of new classes of phenomena, of which it alone can render a satisfactory account.

Newton, who was familiar with the idea of an ether, and who introduced it in some of his speculations, objected, as already stated, that if light consisted of waves shadows could not exist; for that the waves would bend round the edges of opaque bodies and agitate the ether behind them.  He was right in affirming that this bending ought to occur, but wrong in supposing that it does not occur.  The bending is real, though in all ordinary cases it is masked by the action of interference.  This inflection of the light receives the name of Diffraction.