168. A ray of light is a continued stream of these particles, flowing
from any visible body in straight lines. That they move in straight, and
not in crooked lines, unless they be refracted, is evident from bodies
not being visible if we endeavour to look at them through the bore of a
bended pipe; and from their ceasing to be seen by the interposition of
other bodies, as the fixed Stars by the interposition of the Moon and
Planets, and the Sun wholly or in part by the interposition of the Moon,
Mercury, or Venus. And that these rays do not interfere, or jostle one
another out of their ways, in flowing from different bodies all around,
is plain from the following Experiment. Make a little hole in a thin
plate of metal, and set the plate upright on a table, facing a row of
lighted candles standing by one another; then place a sheet of paper or
pasteboard at a little distance from the other side of the plate, and
the rays of all the candles, flowing through the hole, will form as many
specks of light on the paper as there are candles before the plate, each
speck as distinct and large, as if there were only one candle to cast
one speck; which shews that the rays are no hinderance to each other in
their motions, although they all cross in the hole.
[Sidenote: Fig. XI.
In what proportion light and heat decrease at any given
distance from the Sun.