.

Spheroidal waves in crystals, 63.

Spheroids, lemma about, 103.

Sound, speed of, 7, 10, 12.

Telescopes, lenses for, 62, 105.

Torricelli's experiment, 12, 30.

Transparency, explanation of, 28, 31, 32.

Waves, no regular succession of, 17.

Waves, principle of wave envelopes, 19, 24.

Waves, principle of elementary wave fronts, 19.

Waves, propagation of light as, 16, 63.

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