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 - Of Burning Glasses

Chapter 12