, or Translucency, in Physics, a quality in certain bodies, by which they give passage to the rays of light.

The Transparency of natural bodies, as glass, water, air, &c, is ascribed by some, to the great number and size of the pores or interstices between the particles of those bodies. But this account is very defective; for the most solid and opaque body in nature, that we know of, contains a great deal more pores than it does matter; surely a great deal more than is necessary for the passage of so very fine and subtle a body as light.

Aristotle, Des Cartes, &c, make. Transparency to consist in straightness or rectilineal direction of the pores; by means of which, say they, the rays can make their way through, without striking against the solid parts, and so being reflected back again. But this account, Newton shews, is imperfect; the quantity of pores in all bodies being sufficient to transmit all the rays that fall upon them, however those pores be situated with respect to each other.

The reason then why all bodies are not Transparent, is not to be ascribed to their want of rectilineal pores; but either to the unequal density of the parts, or to the pores being filled with some foreign matters, or to their being quite empty, by means of which the rays, in passing through, undergoing a great variety of reflections and refractions, are perpetually diverted different ways, till at length falling on some of the solid parts of the body, they are extinguished and absorbed.

Thus cork, paper, wood, &c, are opake; while glass, diamonds, &c, are Transparent; and the reason is, that in the neighbourhood of parts equal in density with respect to each other, as these latter bodies, the attraction being equal on every side, no reflection or refraction ensues: but the rays which entered the first surface of the body proceed quite through it without interruption, those few only excepted that chance to meet with the solid parts: but in the neighbourhood of parts that differ much in density, such as the parts of wood and paper are, both in respect of themselves and of the air, or the empty space in their pores; as the attraction is very unequal, the reflections and refractions must be very great; and therefore the rays will not be able to make their way through such bodies, but will be variously deflected, and at length quite stopped. See Opacity.

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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