# Letters concerning the English Nation/Letter XVI

LETTER XVI.
ON
Sir Isaac Newton's
OPTICKS.
THE Philosophers of the last Age found out a new Universe; and a Circumstance which made its Discovery more difficult, was, that no one had so much as suspected its Existence. The most Sage and Judicious were of Opinion, that 'twas a frantic Rashness to dare so much as to imagine that it was possible to guess the Laws by which the celestial Bodies move, and the manner how Light acts. ​Galileo by his astronomical Discoveries, Kepler by his Calculation, Des Cartes (at least in his Dioptricks) and Sir Isaac Newton in all his Works, severally saw the Mechanism of the Springs of the World. The Geometricians have subjected Infinity to the Laws of Calculation. The Circulation of the Blood in Animals, and of the Sap in Vegetables, have chang'd the Face of Nature with regard to us. A new kind of Existence has been given to Bodies in the Air-Pump. By the Assistance of Telescopes Bodies have been brought nearer to one another. Finally, the several Discoveries which Sir Isaac Newton has made on Light, are equal to the boldest Things which the Curiosity of Man could expect, after so many philosophical Novelties.
Till Antonio de Dominis, the Rainbow was consider'd as an inexplicable Miracle. This Philosopher guess'd that it was a necessary Effect of the Sun and Rain. Des Cartes gain'd immortal Fame, by his mathematical Explication of this ​so natural a Phænomenon. He calculated the Reflexions and Refractions of Light in Drops of Rain; and his Sagacity on this Occasion was at that Time look'd upon as next to divine.
But what would he have said had it been prov'd to him that he was mistaken in the Nature of Light; that he had not the least Reason to maintain that 'tis a globular Body: That 'tis false to assert, that this Matter spreading it self through the whole, waits only to be projected forward by the Sun, in order to be put in Action, in like Manner as a long Staff acts at one end when push'd forward by the other. That light is certainly darted by the Sun; in fine, that Light is transmitted from the Sun to the Earth in about seven Minutes, tho' a Cannon Ball, which were not to lose any of its Velocity, cou'd not go that Distance in less than twenty five Years. How great wou'd have been his Astonishment, had he been told, that Light does not reflect directly by impinging against the solid Parts of ​Bodies; that Bodies are not transparent when they have large Pores; and that a Man should arise, who would demonstrate all these Paradoxes, and anatomize a single Ray of Light with more Dexterity than the ablest Artist dissects a human Body. This Man is come. Sir Isaac Newton has demonstrated to the Eye, by the bare Assistance of the Prism, that Light is a Composition of colour'd Rays, which, being united, form white Colour. A single Ray is by him divided into seven, which all fall upon a Piece of Linen, or a Sheet of white Paper, in their Order one above the other, and at unequal Distances. The first is Red, the second Orange, the third Yellow, the fourth Green, the fifth Blue, the sixth Indigo, the seventh a Violet Purple. Each of these Rays transmitted afterwards by an hundred other Prisms, will never change the Colour it bears; in like Manner as Gold, when completely purg'd from its Dross, will never change afterwards in the Crucible. As a superabundant Proof that ​each of these elementary Rays has inherently in it self that which forms its Colour to the Eye, take a small Piece of yellow Wood for Instance, and set it in the Ray of a red Colour, this Wood will instantly be ting'd red; but set it in the Ray of a green Colour, it assumes a green Colour, and so of all the rest.
From what Cause therefore do Colours arise in Nature? 'Tis nothing but the Disposition of Bodies to reflect the Rays of a certain Order, and to absorb all the rest.
What then is this secret Disposition? Sir Isaac Newton demonstrates, that 'tis nothing more than the Density of the small constituent Particles of which a Body is compos'd. And how is this Reflexion perform'd? 'Twas suppos'd to arise from the Rebounding of the Rays, in the same Manner as a Ball on the Surface of a solid Body; but this is a Mistake, for Sir Isaac taught the astonish'd Philosophers, that Bodies are opake for no other Reason, but because their Pores are large; that Light reflects ​on our Eyes from the very Bosom of those Pores; that the smaller the Pores of a Body are, the more such a Body is transparent. Thus Paper which reflects the Light when dry, transmits it when oil'd, because the Oil, by filling its Pores, makes them much smaller.
'Tis there that examining the vast Porosity of Bodies, every Particle having its Pores, and every Particle of those Particles having its own; he shows we are not certain that there is a cubic Inch of solid Matter in the Universe, so far are we from conceiving what Matter is. Having thus divided, as it were. Light into its Elements, and carried the Sagacity of his Discoveries so far, as to prove the Method of distinguishing compound Colours from such as are primitive; he shews, that these elementary Rays separated by the Prism, are rang'd in their Order for no other Reason but because they are refracted in that very Order; and 'tis this Property (unknown till he discover'd it) of breaking or splitting in this Proportion; 'tis this unequal Refraction of ​Rays, this Power of refracting the red less than the orange Colour, &c. which he calls the different Refrangibility. The most reflexible Rays are the most refrangible, and from hence he evinces that the same Power is the Cause both of the Reflection and Refraction of Light.
But all these Wonders are merely but the Opening of his Discoveries. He found out the Secret to see the Vibrations or Fits of Light, which come and go incessantly, and which either transmit Light or reflect it according to the Density of the Parts they meet with. He has presum'd to calculate the Density of the Particles of Air necessary between two Glasses, the one flat, the other convex on one side, set one upon the other; in order to operate such a Transmission or Reflexion, or to form such and such a Colour.
From all these Combinations he discovers the Proportion in which Light acts on Bodies, and Bodies act on Light.
He saw Light so perfectly, that he has determin'd to what Degree, of ​Perfection the Art of Increasing it, and of assisting our Eyes by Telescopes can be carried.
Des Cartes, from a noble Confidence, that was very excusable considering how strongly he was fir'd at the first Discoveries he made in an Art which he almost first found out; Des Cartes, I say, hop'd to discover in the Stars, by the Assistance of Telescopes, Objects as small as those we discern upon the Earth.
But Sir Isaac has shown, that Dioptric Telescopes cannot be brought to a greater Perfection; because of that Refraction, and of that very Refrangibilty, which at the same Time that they bring Objects nearer to us, scatter too much the elementary Rays; he has calculated in these Glasses the Proportion of the scattering of the red and of the blue Rays; and proceeding so far as to demonstrate Things which were not suppos'd even to exist, he examines the Inequalities which arise from the Shape or Figure of the Glass, and that which arises from the Refrangibility. He finds, that the object Glass of the Telescope being ​convex on one side and flat on the other, in case the flat Side be turn'd towards the Object, the Error which arises from the Construction and Position of the Glass, is above five thousand Times less than the Error which arises from the Refrangibility: And therefore, that the Shape or Figure of the Glasses is not the Cause why Telescopes cannot be carried to a greater Perfection, but arises wholly from the Nature of Light.
For this Reason he invented a Telescope, which discovers Objects by Reflection and not by Refraction. Telescopes of this new kind are very hard to make, and their Use is not easy. But according to the English, a reflective Telescope of but five Feet, has the same Effect as another of an hundred Feet in Length.
This work was published before January 1, 1929, and is in the public domain worldwide because the author died at least 100 years ago.
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