the
garden fronting the window there stands a tree. Now if the rays of light
which pass from the tree through the hole in the window-shutter are
allowed to fall upon a screen in the darkened room, it will be found
that the image is inverted.

This is accounted for by the fact, that the rays cross each other at the
hole, and proceeding in straight lines, form an inverted picture on the
screen. It can further be proved, that the path of a ray of light
through space as it proceeds from the sun is also that of a straight
line. Whenever there is a solar eclipse we have light so long as we can
see the smallest part of the sun's surface. The instant, however, that
we have a total eclipse, at that instant the whole of the light of the
sun is shut off, and for a brief space there is darkness, until the
planet which is causing the eclipse has passed on in its orbit and the
sun's surface reappears again. Now if light did not proceed in straight
lines, such an event as a total eclipse would be impossible; because, if
the light proceeded in curved lines instead of straight ones from the
sun, then even when the planet which causes the eclipse got directly
between the earth and the sun, the rays of light being curved instead
of straight would bend round the eclipsing planet, and so would not all
be intercepted, and thus such an event as a total eclipse would be an
impossibility. From this we learn, therefore, that the path of a ray of
light as it proceeds from the sun through space is that of a straight
line, and that the path corresponds to the radius vector of a circle,
which is also the path that the centripetal force takes.

Viewing the matter from the standpoint of the solar system, we find the
sun, which is the centre of that system, exerting an attractive force
along the radius vector of all the orbits of the planets, with a force
which decreases in intensity inversely as the square of the distance. At
the same time, being the source of all light, it is constantly
propagating into space aetherial light waves with a velocity almost
inconceivable; which also decrease in exactly the same ratio as the
attractive power of the sun decreases. If, therefore, it can be shown
that there is such a truth as the dynamical value of light, in the same
way that it has been shown that there is a dynamical value of heat, then
it follows, that not only is the sun the centre of an attractive power
which proceeds in straight lines, but it is equa