Page:Scientific Papers of Josiah Willard Gibbs.djvu/441

Rh any influence upon the intensity of the radiations passing through the spaces between and around them; since the heat reflected by a screen in any direction is the exact equivalent of that which would proceed in the same direction (without reflection) if the screen were not there. So, also, the heat passing through any aperture in a screen is the exact equivalent of that which would be reflected in the same direction if there were no aperture. The quantities of radiant heat which fall upon the bodies $$\text{A}$$ and $$\text{B}$$ are therefore entirely unchanged by the presence and the motion of the screens, and their temperature cannot be affected.

We may conclude a fortiori that $$\text{B}$$ will not grow warmer if $$\text{A}$$ is colder than $$\text{B}$$, and none of the other bodies present are warmer than $$\text{B}$$.

Since the body $$\text{B}$$, for example, when the screens are in motion, does not receive radiations from every body to which it sends them, it is not without interest to inquire from what bodies it will receive its share of heat. This problem may be solved most readily by supposing the screens to move in the opposite direction, with the same velocity as before. One may easily convince himself that every body which receives radiant heat from A when the apparatus moves backward, will impart heat to $$\text{A}$$ when the apparatus moves forward, and to exactly the same amount, if its temperature is the same as that of $$\text{A}$$.