351. Reflexion is no repulsion of light, but only its tension continued into the medium, in which the tension has been. 352. A non-transparent body indicates nothing for the tension of light but the limit of the co-illuminating matter; it does not at all operate itself upon the light, it is as it were a void space. 353. Transparent bodies also reflect partly, because they are only relative æther, because they only co-illuminate, are not themselves tensed; or because the basis in all is the metal. Every other medium is, however, an æther differently fixed; in every one therefore the tension has been changed; every medium is thus a limit for the tension, and therefore the transparent bodies also reflect. Since the tension becomes altered, when it passes into another medium, it always remains by preference in the neutral medium; therefore reflexion originates also by the air, when the light passes out of glass very obliquely into it. _Operation of the terrestrial Elements upon Light._ DECOMPOSITION OF LIGHT--COLOURS. 354. It results from all this, that light cannot enter unchanged into mutual operation with matter. The tension of æther changes itself in matter. This change of light through the influence of matter is a debilitation of the tension of æther and lastly its complete cessation. There can be therefore no absolutely transparent matter; the æther only is this absolutely transparent matter. The denser a material is, by so much the more will it be capable of suppressing in itself the tension of light. The most transparent bodies must also become with a greater density adiaphanous or opaque, because the Metallic increases in them. 355. This suppression or expiration of the tension of light in bodies has received, as likewise proceeding from the mechanical theory of light, the name of _absorption_. The absorption is not a mechanical adherence of the particles of light in the pores of bodies. There are no pores for light, and this requires none. 356. The absorption or _decrescence_ of light is a retrogression of light into the indifference of æther, into _darkness_. Light in conflict with matter does not continue light, but becomes a mean condition between light and darkness. 357. The substratum of light, the æther, has two extreme conditions, and only two, the tensed and the non-tensed; the one is the _light_, the other the _dark_. Between the two, however, are the mediate conditions of _twilight_ or "clare-obscure." The light æther emits rays, the dark does not; the mediate conditions are half the two. The light condition is the clear unsullied light, the absolute translucency; the darkness is the absolute want of translucency; the mediate members are offuscated light, mediate tensions of æther. 358. The mean tension of æther, or light mingled with darkness, is called _Colour_. Colour is a finite, fixed light, the actual transmission of light into matter. 359. No matter can be uncoloured. An uncoloured matter is a nonentity.