The tube feet of sea urchins contain photoreceptor proteins, which together act as a compound eye; they lack screening pigments, but can detect the directionality of light by the shadow cast by its opaque body.
12.
In a cavity in an opaque body with rigid walls that are not perfectly reflective at any frequency, in thermodynamic equilibrium, there is only one temperature, and it must be shared in common by the radiation of every frequency.
13.
And since the air too admits of influence and undergoes every kind of change, clouds form in it . rain-storms beat down, and snow, hoar-frost, hail with blasts of winds and of hurricanes, and thunder too and lightning and falling bolts, and the crashing together of countless opaque bodies ."
14.
In my opinion, sitting in a movie theater watching a computer-simulated alien invasion of Earth can't hold a candle to the sensation of slipping down a steep bank toward an opaque body of water full of an undetermined number of dangerous ( italics ) Chelydra serpentina . ( end italics)
15.
In slightly different terms, the emissive power of an arbitrary opaque body of fixed size and shape at a definite temperature can be described by a dimensionless ratio, sometimes called the emissivity, the ratio of the emissive power of the body to the emissive power of a black body of the same size and shape at the same fixed temperature.
