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CHAPTER XI
NEWTON’S THEORY OF COLOURS, INDICATED BY PYTHAGORAS
AND PLATO.
1. THAT the wonderful theory, whereby is investigated and
distinguished from one another, all that variety of colours, which
enters into the composition of that uniform appearance, light,
might of itself suffice to establish forever the glory of Sir
Isaac Newton, and be an eternal monument of the extraordinary
sagacity of that great man. That discovery seems. by its importance,
to have been reserved for an age when philosophy had arrived at
its fullest maturity ; and yet it is to be found among some of
the eminent men of the first antiquity, whose genius had no occasion
for the experience of many ages to form it, as is strikingly evident
from their having given birth to the sciences. Of this number
are Pythagoras and Plato. The former of whom, and. his disciples
after him, entertained sufficiently just conceptions of the formation
of colours. They taught that they resulted from the differ-. ent
modifications of reflected light; or as a modern author, in explain.
ing the sentiments of the Pythagoreans, expresses it, light reflecting
itself with more or less vivacity, forms by that means our different.
sensations of colour. Those same philosophers of the Pythagoric
school, in assigning the reason of the difference in colours,
ascribe it to a mixture of the elements of light ; and divesting
the atoms, or small particles of light, of all manner of colour,
impute every sensation of that kind to the motions excited in
our organs of sight. The disciples of Plato contributed not a
little to the advancement of optics, by the important discovery
they made, that light emits itself in straight lines, and that
the angle of incidence is always equal to the angle of reflection.
2. Plato also seems to have apprehended the
Newtonian system of colours; for he calls them the effect of light
transmitted from bodies, the small particles of which were adapted
to the organs of sight. Now is not this precisely the same with
what Sir Isaac teaches, “that the different sensations of
each particular colour are excited in us by the difference of
size in those small particles of light which form the several
rays; those small particles occasioning different images of colour,
as the vibration is more or less lively with which they strike
our sense?” The same philosopher hath gone further: lie bath
entered into a detail of the composition of colours, and inquired
into the visible effects that must arise from a mixture of the
different rays of which light itself is composed. And what he
advances a little farther on, that it was not in the power of
man exactly to determine what the proportion of this mixture should
be in certain colours, sufficiently shows, that lie bad an idea
of this theory, though he judged it almost impossible to unfold
it; which makes him add, that “should any one arrive at the
knowledge of this proportion, he ought not to hazard the discovery
of it, since it would be impossible to demonstrate it by clear
and convincing proofs ;“ and yet he thought “certain
rules might be laid down” respecting this subject “if
in following and imitating nature, we could arrive at the art
of forming a diversity of colours, by the combined intermixture
of colours.” And he afterward adds, what may be regarded
as the noblest eulogium that ever was made on Sir Isaac Newton:
“yea, should ever any one,” exclaims that fine genius
of antiquity, “attempt by curious research to account for
this admirable mechanism, he will, in doing so, but manifest how
entirely ignorant he is of the difference between Divine and human
power. It is true, God can intermingle those things one with another,
and then sever them at his pleasure, because he is, at the same
time, all-knowing and all. powerful; but there is no man now exists,
nor ever will perhaps, who shall ever be able to accomplish things
so very difficult.” What an eulogimn are these words in the
mouth of such a philosopher as Plato, and bow glorious is he who
hath successfully accomplished what appeared impracticable to
that prince of philosophers! And what elevation of genius, what
piercing penetration into the most intimate secrets of nature,
displays itself in what we have just now recited from Plato, concerning
the nature and theory of colours, at a time when philosophy was
but yet in its infancy.
3. Although the system of Descartes, respecting
the propagation of light in an instant, is scarcely admitted at
present by the most part of philosophers, nor has been ever since
Messrs. Cassini and Romer discovered that its motion was progressive;
yet as that system was for a long while in vogue, and the whole
honour of the invention of it ascribed to Descartes, it will
not be amiss, in a few words, to make appear, that he drew the
idea of it from Aristotle and his commentators. The opinion of
the modern philosophers is, that light is nothing else but the
action of a subtile matter upon the organs of sight. This subtile
matter is supposed to fill all that space which lies between
the sun and us; and that particle of it, which is next to the
sun, receiving thence an impulse, must instantaneously communicate
it to all the rest which lie between the sun and the organ of
sight. To render this the more evident, Descartes introduces the
comparison of a stick; which, by reason of the continuity of its
parts, cannot in any degree be moved lengthways at one end, without
instantaneously being put into the same degree of motion at the
other end.
4. Whoever will be at the pains attentively
to read what Aristotle hath written concerning light, without
having recourse to the ridiculous interpretations that have been
put upon his words, will clearly discern, that he was far from
being so unacquainted with the truth in this case, as is generally
thought. He defines it to be the action of a subtile, pure, and
homogeneous matter; and Philoponus explaining the mariner in which
this action was performed, makes use of the instance of a long
string, which being pulled at one end, will instantaneously be
moved at the other. In that very place, he resembles the sun,
to the man who pulls the string; the subtile matter, to the string
itself; and the instantaneous action of the one, to the movement
of the other. Simphicius, in his commentary upon this passage
of Aristotle, expressly employs the motion of a stick, to imitate
how light, acted upon the sun, may instantaneously impress the
organs of sight. The comparison of a stick, to convey an idea
of the celerity with which light may communicate itself, seems
first of all to have been made use of by Chrysippus.
Chapter 12
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