... phenomenal is light. Unlike sound-waves, whose transmission requires air or other material media, light-waves pass freely through the vacuum of interstellar space. Even the hypothetical ether, held as the interplanetary medium of light in the undulatory theory, can be discarded on the Einsteinian grounds that the geometrical properties of space render the theory of ether unnecessary. Under either hypothesis, light remains the most subtle, the freest from material dependence, of ... — Autobiography of a YOGI • Paramhansa Yogananda
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