That wonderful theory, whereby is investigated and distinguished from
one another the variety of colours that constitute the uniform
appearance, called light, establishes the glory of sir Isaac Newton, and
is an eternal monument of his extraordinary sagacity. Its discovery was
reserved for an age when philosophy had arrived at its fullest maturity;
and yet it is to be found in the writings of some of the most eminent
men of ancient times.
Pythagoras, and his disciples after him, entertained sufficiently just
conceptions of the formation of colours. They taught that “they resulted
solely from the different modification of reflected light;” or, as a
modern author, in explaining the sentiments of the Pythagoreans,
expresses it, “light reflecting itself with more or less vivacity,
forms by that means our different sensations of colour.” The same
philosophers, “in assigning the reason of the difference of 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.
Plato terms colours “the effect of light transmitted from bodies, the
small particles of which were adapted to the organ of sight.” This seems
precisely what sir Isaac Newton teaches in his “Optics,” viz. 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.” But the _ancient_ philosopher went further. He 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.” He advances, however, that “it is not in the
power of man exactly to determine what the proportion of this mixture
should be in certain colours.” This sufficiently shows, that he had an
idea of this theory, though he judged it almost impossible to unfold it.
He says, 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 others.”
It is to be remarked, that Plato adds what may be regarded as
constituting the noblest tribute that can be offered in praise of 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, that
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 eulogium is this from the pen of Plato! How glorious is he who
has successfully accomplished what appeared impracticable to the prince
of ancient philosophers! Yet what elevation of genius, what piercing
penetration into the most intimate secrets of nature, displays itself in
these passages concerning the nature and theory of colours, at a time
when Greek philosophy was in its infancy!
* * * * *
LIGHT--Aristotle and Descartes.
Although the system of Descartes, respecting the propagation of light in
an instant, has been discarded since Cassini discovered that its motion
is progressive; yet it may not be amiss to show from whence he obtained
the idea. His opinion was, that light is the mere action of a subtile
matter upon the organs of sight. This subtile matter he supposes to fill
all that space which lies between the sun and us; and that the particle
of it, which is next to the sun, receiving thence an impulse,
instantaneously communicates it to all the rest, between the sun and the
organ of sight. To evidence this, 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. Whoever will be at the
pains to read, attentively, what Aristotle hath written concerning
light, will perceive that he defines it to be the action of a subtile,
pure, and homogeneous matter. Philoponus, explaining the manner 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: he resembles the sun, to the man who quills the string; the
subtile matter, to the string itself; and the instantaneous action of
the one, to the movement of the other. Simplicius, in his commentary
upon this passage of Aristotle, expressly employs the motion of a stick,
to intimate how light, acted upon by the sun, may instantaneously
impress the organs of sight. This comparison of a stick seems to have
been made use of first, by Chrysippus--lastly, by Descartes.
* * * * *
~Durhamiana.~
_For the Table Book._