Sir Isaac Newton’s Theory of Colours indicated by Pythagoras and Plato.
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.