f colored rays,
which, being united, form white color. A single ray is by him divided
into seven, which all fall upon a piece of linen or a sheet of white
paper, in their order one above the other, and at equal distances. The
first is red, the second orange, the third yellow, the fourth green, the
fifth blue, the sixth indigo, the seventh a violet purple. Each of these
rays transmitted afterwards by a hundred other prisms will never change
the color it bears; in like manner as gold, when completely purged from
its dross, will never change afterwards in the crucible."(3)

XII. NEWTON AND THE LAW OF GRAVITATION

We come now to the story of what is by common consent the greatest of
scientific achievements. The law of universal gravitation is the most
far-reaching principle as yet discovered. It has application equally
to the minutest particle of matter and to the most distant suns in the
universe, yet it is amazing in its very simplicity. As usually phrased,
the law is this: That every particle of matter in the universe attracts
every other particle with a force that varies directly with the mass
of the particles and inversely as the squares of their mutual distance.
Newton did not vault at once to the full expression of this law,
though he had formulated it fully before he gave the results of his
investigations to the world. We have now to follow the steps by which he
reached this culminating achievement.

At the very beginning we must understand that the idea of universal
gravitation was not absolutely original with Newton. Away back in
the old Greek days, as we have seen, Anaxagoras conceived and clearly
expressed the idea that the force which holds the heavenly bodies
in their orbits may be the same that operates upon substances at the
surface of the earth. With Anaxagoras this was scarcely more than a
guess. After his day the idea seems not to have been expressed by any
one until the seventeenth century's awakening of science. Then the
consideration of Kepler's Third Law of planetary motion suggested to
many minds perhaps independently the probability that the force hitherto
mentioned merely as centripetal, through the operation of which the
planets are held in their orbits is a force varying inversely as the
square of the distance from the sun. This idea had come to Robert Hooke,
to Wren, and perhaps to Halley, as well as to Newton; but as yet no one
had conceived a method by which the validity of the suggest