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Suddenly a small white hand swept the horizon with a swift, undulatory motion that reminded Darrell of the flight of some white-winged bird, and Kate cried, "Did we think of the roughness and steepness of the path below when we stood here two hours ago and looked on the glory of this scene?
In form she was swan-like, undulatory, so to speak. Her features were prononce; nose, aquiline; eyes, piercing; hair, black as night, and in long ringlets. Miss Flouncer was, as I have said, an elderly spinster. Sir Richard was an elderly bachelor. Miss Flouncer thought of this, and often sighed.
To-day, a boy who does not know that one may talk very agreeably with a friend a thousand miles away is an ignoramus; and experimenters whisper among themselves that, if the undulatory theory of light have any foundation, there is no real reason why we may not see that same friend at that same distance, as well as talk with him.
The best instances for the determination of this question are those in which two different explanations have been given of the same thing. The most celebrated case of this kind is that of the corpuscular and the undulatory theories of light. Up to a certain point the phenomena of light are equally well explained by both; beyond this point, one of them fails.
But, while I have thus endeavoured to illustrate before you the power of the undulatory theory as a solver of all the difficulties of optics, do I therefore wish you to close your eyes to any evidence that may arise against it? By no means.
What is to be thought of the contemporary of Young and of Fresnel, who never misses an opportunity of casting scorn upon the hypothesis of an ether the fundamental basis not only of the undulatory theory of light, but of so much else in modern physics and whose contempt for the intellects of some of the strongest men of his generation was such, that he puts forward the mere existence of night as a refutation of the undulatory theory?
Darwin's hypothesis, therefore, subject to the production of proof that physiological species may be produced by selective breeding; just as a physical philosopher may accept the undulatory theory of light, subject to the proof of the existence of the hypothetical ether; or as the chemist adopts the atomic theory, subject to the proof of the existence of atoms; and for exactly the same reasons, namely, that it has an immense amount of prima facie probability: that it is the only means at present within reach of reducing the chaos of observed facts to order; and lastly, that it is the most powerful instrument of investigation which has been presented to naturalists since the invention of the natural system of classification, and the commencement of the systematic study of embryology.
My vulture is also a bird of leisure, and sails through the ether on long flexible pinions, as if that was the one delight of his life. Some birds have wings, others have "pinions." The buzzard enjoys this latter distinctions. There is something in the sound of the word that suggests that easy, dignified, undulatory movement.
These "diffraction rings" arise from the undulatory nature of light, and their distance apart as well as the diameter of the central disk depend upon the length of the waves of light.
Fresnel freely acknowledged Young's priority so soon as his attention was called to it; and Young applauded the work of the Frenchman, and aided with his counsel in the application of the undulatory theory to the problems of polarization of light, which still demanded explanation, and which Fresnel's fertility of experimental resource and profundity of mathematical insight sufficed in the end to conquer.
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