What's the difference between diacaustic and diacoustic?

Diacaustic

Definition:

• (a.) Pertaining to, or possessing the properties of, a species of caustic curves formed by refraction. See Caustic surface, under Caustic.
• (n.) That which burns by refraction, as a double convex lens, or the sun's rays concentrated by such a lens, sometimes used as a cautery.
• (n.) A curved formed by the consecutive intersections of rays of light refracted through a lens.

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Diacoustic

Definition:

• (a.) Pertaining to the science or doctrine of refracted sounds.

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