Sir Isaac Newton’s Optics

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 it was 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, Descartes (at least, in his dioptrics), 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 changed 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.

The philosophers of the previous century discovered a new universe. The discovery was all the harder because no one had even suspected that such a universe existed. The wisest and most thoughtful people considered even imagining such discoveries an act of reckless madness. Could anyone possibly work out the laws governing the movement of celestial bodies or the behavior of light? Galileo, through his astronomical discoveries, and Kepler, through his calculations, each saw how the world’s machinery worked. So did Descartes, at least in his work on optics, and Sir Isaac Newton, in all his works. Mathematicians have brought infinity under the laws of calculation. Discoveries about the circulation of blood in animals and sap in plants have transformed our view of Nature. The air pump has given bodies a new kind of existence. Telescopes have brought bodies closer to one another. Finally, Sir Isaac Newton’s discoveries about light match the boldest hopes human curiosity could entertain after so many scientific breakthroughs.

Till Antonio de Dominis the rainbow was considered as an inexplicable miracle. This philosopher guessed that it was a necessary effect of the sun and rain. Descartes gained immortal fame by his mathematical explication of this so natural a phenomenon. He calculated the reflections and refractions of light in drops of rain. And his sagacity on this occasion was at that time looked upon as next to divine.

Before Antonio de Dominis, the rainbow was considered an unexplained miracle. This philosopher guessed that rainbows were a necessary result of the sun and rain. Descartes won lasting fame for his mathematical explanation of this entirely natural phenomenon. He calculated how light was reflected and refracted in raindrops. At the time, his insight into rainbows was seen as almost divine.

But what would he have said had it been proved to him that he was mistaken in the nature of light; that he had not the least reason to maintain that it is a globular body? That it is false to assert that this matter, spreading itself 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 pushed 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, though a cannonball, which were not to lose any of its velocity, could not go that distance in less than twenty-five years. How great would 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 anatomise 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 coloured 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 a hundred other prisms, will never change the colour it bears; in like manner, as gold, when completely purged from its dross, will never change afterwards in the crucible. As a superabundant proof that each of these elementary rays has inherently in itself 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 tinged red. But set it in the ray of a green colour, it assumes a green colour, and so of all the rest.

But what would he have said if someone had proved him wrong about the nature of light? What if he had been shown he had no reason at all to claim that light consists of spherical bodies? Or that he was wrong to say that light is matter spread throughout space, waiting for the sun to activate it? In his view, the sun only needed to push this matter forward. A long rod works the same way: push one end, and the other moves. What if he had learned that the sun certainly sends light shooting out? Finally, what if he had learned that light travels from the sun to the earth in about seven minutes? A cannonball, even if it never lost any speed, could not cover that distance in less than twenty-five years. How astonished he would have been to learn that light does not reflect simply by striking the solid parts of bodies. Or that large pores do not make bodies transparent. What if he had been told that someone would come along and prove all these seemingly impossible claims? This man would dissect a single ray of light more skillfully than the finest anatomist dissects a human body. That man has arrived. Sir Isaac Newton has shown, using only a prism, that light consists of coloured rays that combine to form white. He divides a single ray into seven. These rays fall on a piece of linen or a sheet of white paper in order. They appear one above another, at unequal distances. The first is red, the second orange, the third yellow, the fourth green, and the fifth blue. The sixth is indigo, and the seventh violet-purple. Pass each ray through a hundred more prisms, and its colour will never change. In the same way, gold completely purified of impurities will never change again in the crucible. Here is further proof, beyond what is needed, that each basic ray inherently contains what gives it its visible colour. Take a small piece of yellow wood, for example, and place it in the red ray. The wood will instantly appear red. But place the wood in the green ray, and it turns green. The same holds for all the other rays.

From what cause, therefore, do colours arise in Nature? It is nothing but the disposition of bodies to reflect the rays of a certain order and to absorb all the rest.

So what causes colours in nature? Colours arise only from the way objects reflect certain kinds of light rays and absorb all the rest.

What, then, is this secret disposition? Sir Isaac Newton demonstrates that it is nothing more than the density of the small constituent particles of which a body is composed. And how is this reflection performed? It was supposed 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 astonished philosophers that bodies are opaque 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 oiled, because the oil, by filling its pores, makes them much smaller.

What, then, is this hidden property? Sir Isaac Newton shows that it is simply the density of the tiny particles that make up a body. And how does this reflection happen? People thought light rays bounced back like a ball bouncing off a solid surface. But this is a mistake. Sir Isaac astonished philosophers by teaching that bodies are opaque only because their pores are large. Light reflects into our eyes from deep inside those pores. The smaller a body's pores, the more transparent the body is. Thus paper reflects light when dry but lets light pass through when oiled. This is because the oil fills the paper's pores, making them much smaller.

It is 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 shows that these elementary rays, separated by the prism, are ranged in their order for no other reason but because they are refracted in that very order; and it is this property (unknown till he discovered it) of breaking or splitting in this proportion; it is 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.

There he examines how porous bodies are. Every particle has pores, and every particle within those particles has pores of its own. He shows that we cannot be certain there is even a cubic inch of solid matter in the universe. So far are we from understanding what matter is. Having thus divided light into its elements, so to speak, he takes his insightful discoveries further. He demonstrates how to distinguish compound colours from primary colours. He then shows why the elementary rays separated by a prism appear in their particular order. The rays appear in that order only because they are refracted in that order. This property was unknown until he discovered it. Light rays bend by different amounts, with red rays bending less than orange rays, and so on. He calls this unequal bending of rays their different refrangibility. The rays most easily reflected are also the most easily refracted. From this relationship, he demonstrates that the same force causes both 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 presumed 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 reflection, or to form such and such a colour.

But all these wonders are only the beginning of his discoveries. He discovered how to observe the vibrations or fits of light, which constantly come and go. These vibrations either transmit or reflect light, depending on the density of the matter they encounter. He even ventured to calculate the density of air particles needed between two pieces of glass placed together. One piece is flat, and the other is convex on one side. He calculated the density needed to transmit or reflect light in this way, or to produce a particular colour.

From all these combinations he discovers the proportion in which light acts on bodies and bodies act on light.

From all these combinations, he discovers how strongly light affects bodies and how strongly bodies affect light.

He saw light so perfectly, that he has determined to what degree of perfection the art of increasing it, and of assisting our eyes by telescopes, can be carried.

He understood light perfectly. As a result, he determined how far we can amplify light and improve our vision with telescopes.

Descartes, from a noble confidence that was very excusable, considering how strongly he was fired at the first discoveries he made in an art which he almost first found out; Descartes, I say, hoped to discover in the stars, by the assistance of telescopes, objects as small as those we discern upon the earth.

Descartes had a noble confidence, quite excusable given his excitement over his first discoveries. Those discoveries were in a field he had very nearly founded himself. Descartes, I say, hoped telescopes would reveal objects among the stars as small as those we see on earth.

But Sir Isaac has shown that dioptric telescopes cannot be brought to a greater perfection, because of that refraction, and of that very refrangibility, 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 supposed 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 turned 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.

But Sir Isaac has shown that refracting telescopes cannot be improved further because of refraction and differences in refrangibility. These properties bring objects closer to us, but also spread the individual rays too widely. He has calculated the relative spread of red and blue rays in these lenses. Going so far as to demonstrate effects no one had even supposed existed, he examines two sources of distortion. One comes from the shape of the glass, and the other from differences in refrangibility. He considers a telescope’s objective lens that is convex on one side and flat on the other. If the flat side faces the object, the error caused by the lens’s construction and position is comparatively tiny. The error is more than five thousand times smaller than the error caused by differences in refrangibility. Therefore, the shape of the lenses is not what prevents telescopes from being improved further. The limitation comes entirely 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 a hundred feet in length.

For this reason, he invented a telescope that reveals objects through reflection rather than refraction. These new telescopes are very hard to make and difficult to use. But according to the English, a reflecting telescope only five feet long works as well as a hundred-foot refracting telescope.