s upon them, in which case the light is "stopped or stifled"
by the multitude of reflections.

The _colours_ of natural bodies have, in the Newtonian hypothesis, the
same origin as the colours of thin plates, their transparent particles,
according to their several sizes, reflecting rays of one colour and
transmitting those of another.

Among the optical discoveries of Newton those which he made on the
inflection of light hold a high place. They were first published in his
"Treatise on Optics," in 1707.

_III--The Discovery of the Law of Gravitation_

From the optical labours of Newton we now proceed to the history of his
astronomical discoveries, those transcendent deductions of human reason
by which he has secured to himself an immortal name, and vindicated the
intellectual dignity of his species.

In the year 1666, Newton was sitting in his garden at Woolsthorpe,
reflecting on the nature of gravity, that remarkable power which causes
all bodies to descend towards the centre of the earth. As this power
does not sensibly diminish at the greatest height we can reach he
conceived it possible that it might reach to the moon and affect its
motion, and even hold it in its orbit. At such a distance, however, he
considered some diminution of the force probable, and in order to
estimate the diminution, he supposed that the primary planets were
carried round the sun by the same force. On this assumption, by
comparing the periods of the different planets with their distances from
the sun, he found that the force must decrease as the squares of the
distances from the sun. In drawing this conclusion he supposed the
planets to move in circular orbits round the sun.

Having thus obtained a law, he next tried to ascertain if it applied to
the moon and the earth, to determine if the force emanating from the
earth was sufficient, if diminished in the duplicate ratio of the moon's
distance, to retain the moon in its orbit. For this purpose it was
necessary to compare the space through which heavy bodies fall in a
second at the surface of the earth with the space through which the
moon, as it were, falls to the earth in a second of time, while
revolving in a circular orbit. Owing to an erroneous estimate of the
earth's diameter, he found the facts not quite in accordance with the
supposed law; he found that the force which on this assumption would act
upon the moon would be one-sixth more than required to retain it in its
orbit