Voltaire · Complete work
Letter XVI
Letter XVI of 24. Read it here for reference, or continue through the entire work without leaving the reader.
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sur l’optique de newton.
Un nouvel univers a été découvert par les philosophes du dernier siècle, et ce monde nouveau était d’autant plus difficile à connaître qu’on ne se doutait pas même qu’il existât. Il semblait aux plus sages que c’était une témérité d’oser seulement songer qu’on pût deviner par quelles lois les corps célestes se meuvent et comment la lumière agit.
Galilée, par ses découvertes astronomiques, Kepler, par ses calculs, Descartes, au moins dans sa Dioptrique, et Newton, dans tous ses ouvrages, ont vu la mécanique des ressorts du monde. Dans la géométrie, on a assujetti l’infini au calcul. La circulation du sang dans les animaux et de la sève dans les végétables a changé pour nous la nature. Une nouvelle manière d’exister a été donnée aux corps dans la machine pneumatique. Les objets se sont rapprochés de nos yeux à l’aide des télescopes. Enfin, ce que Newton a découvert sur la lumière est digne de tout ce que la curiosité des hommes pouvait attendre de plus hardi, après tant de nouveautés.
Jusqu’à Antonio de Dominis, l’arc-en-ciel avait paru un miracle inexplicable; ce philosophe devina que c’était un effet nécessaire de la pluie et du soleil. Descartes rendit son nom immortel par l’explication mathématique de ce phénomène si naturel; il calcula les réflexions de la lumière dans les gouttes de pluie, et cette sagacité eut alors quelque chose de divin.
Mais qu’aurait-il dit si on lui avait fait connaître qu’il se trompait sur la nature de la lumière; qu’il n’avait aucune raison d’assurer que c’était un corps globuleux; qu’il est faux que cette matière, s’étendant par tout l’univers, n’attende, pour être mise en action, que d’être poussée par le soleil, ainsi qu’un long bâton qui agit à un bout quand il est pressé par l’autre; qu’il est très vrai qu’elle est dardée par le soleil, et qu’enfin la lumière est transmise du soleil à la terre en près de sept minutes, quoique un boulet de canon, conservant toujours sa vitesse, ne puisse faire ce chemin qu’en vingt-cinq années?
Quel eût été son étonnement si on lui avait dit: « Il est faux que la lumière se réfléchisse directement en rebondissant sur les parties solides du corps; il est faux que les corps soient transparents quand ils ont des pores larges; et il viendra un homme qui démontrera ces paradoxes, et qui anatomisera un seul rayon de lumière avec plus de dextérité que le plus habile artiste ne dissèque le corps humain!
Il a si bien vu la lumière qu’il a déterminé à quel point l’art de l’augmenter et d’aider nos yeux par des télescopes doit se borner.
Descartes, par une noble confiance bien pardonnable à l’ardeur que lui donnaient les commencements d’un art presque découvert par lui, Descartes espérait voir dans les astres, avec des lunettes d’approche, des objets aussi petits que ceux qu’on discerne sur la terre.
Newton a montré qu’on ne peut plus perfectionner les lunettes, à cause de la réfraction même, qui, en nous rapprochant les objets, écartent trop les rayons élémentaires; il a calculé, dans ces verres, la proportion de l’écartement des rayons rouges et des rayons bleus; et, portant démonstration dans des choses dont on ne soupçonnait pas même l’existence, il examine les inégalités que produit la figure du verre, et celle que fait la réfrangibilité. Il trouve que le verre objectif de la lunette étant convexe d’un côté et plat de l’autre, si le côté plat est tourné vers l’objet, le défaut qui vient de la construction et de la position du verre est cinq mille fois moindre que le défaut qui vient par la réfrangibilité; et qu’ainsi ce n’est pas la figure des verres qui fait qu’on ne peut perfectionner les lunettes d’approche, mais qu’il faut s’en prendre à la matière même de la lumière.
Voilà pourquoi il inventa un télescope qui montre les objetspar réflexion, et non point par réfraction.
Il était encore peu connu en Europe quand il fit cette découverte.J'ai vu un petit livre composé environ ce temps-là, danslequel, en parlant du télescope de Newton, on le prend pour unlunetier: Artifex quidam Anglus nomine Newton. La postérité l'abien vengé.
Musean translation
Mouseia’s complete machine-assisted Musean translation, made directly from the French text of all twenty-four letters (Garnier edition, 1879, French Wikisource) for fidelity, the author’s force and cadence, and modern clarity.
On Newton's Optics
A new universe was discovered by the philosophers of the last century, and this new world was all the harder to know because no one even suspected that it existed. To the wisest minds it seemed rash merely to imagine that one could discover the laws by which the heavenly bodies move and how light acts.
Galileo, through his astronomical discoveries, Kepler, through his calculations, Descartes, at least in his Optics, and Newton, throughout his works, have seen the workings of the world's machinery. In geometry, the infinite has been brought under calculation. The circulation of blood in animals and of sap in plants has transformed our conception of nature. A new mode of existence has been given to bodies in the air pump. Telescopes have brought objects closer to our eyes. Finally, what Newton discovered about light is worthy of the boldest hopes human curiosity could entertain after so many new discoveries.
Until Antonio de Dominis, the rainbow had seemed an inexplicable miracle; this philosopher divined that it was a necessary effect of rain and sun. Descartes made his name immortal through the mathematical explanation of this most natural phenomenon; he calculated the reflections of light in raindrops, and his insight then seemed almost divine.
But what would he have said if he had been told that he was mistaken about the nature of light; that he had no reason to assert it was a globular body; that it is false that this matter, spread throughout the universe, merely waits to be set in action by a push from the sun, like a long rod that moves at one end when pressed at the other; that it is quite true that light is projected from the sun, and that light is transmitted from the sun to the earth in nearly seven minutes, although a cannonball, even if it kept its speed throughout, could cover that distance only in twenty-five years?
How astonished he would have been to hear: “It is false that light is reflected directly by rebounding from the solid parts of a body; it is false that bodies are transparent when they have wide pores; and a man will come who will demonstrate these paradoxes and dissect a single ray of light more deftly than the most skillful practitioner dissects the human body!”
He understood light so well that he determined the point beyond which the art of magnifying objects and aiding our eyes with telescopes cannot go.
Descartes, with a noble confidence readily forgiven in view of the ardor inspired by the beginnings of an art he had almost discovered himself—Descartes hoped that telescopes would reveal objects in the stars as small as those we can distinguish on earth.
Newton showed that telescopes cannot be further perfected because refraction itself, though it brings objects nearer to us, spreads their constituent rays too far apart. He calculated the proportion in which red and blue rays spread apart in these lenses; and, extending demonstration to things whose very existence had not even been suspected, he examines the irregularities produced by the shape of the glass and those produced by refrangibility. He finds that, if the objective lens of a telescope is convex on one side and flat on the other, and the flat side faces the object, the defect due to the construction and position of the lens is five thousand times smaller than the defect due to refrangibility; and thus it is not the shape of the lenses that prevents us from perfecting telescopes: the fault lies with the very nature of light.
That is why he invented a telescope that shows objects by reflection and not by refraction.
He was still little known in Europe when he made this discovery. I have seen a little book composed around that time which, speaking of Newton's telescope, takes him for a maker of optical instruments: Artifex quidam Anglus nomine Newton. Posterity has amply avenged him.
Plain English translation
Mouseia’s complete Plain English edition, made independently and directly from the French text of all twenty-four letters (Garnier edition, 1879, French Wikisource).
On Newton's Optics
The philosophers of the last century discovered a new universe. This new world was all the harder to understand because no one had even suspected that it existed. The wisest people thought it rash even to imagine that we could discover the laws that govern how heavenly bodies move and how light works.
Galileo, through his astronomical discoveries, Kepler, through his calculations, Descartes, at least in his Optics, and Newton, in all his works, have seen the mechanics that drive the world. In geometry, infinity has been brought within the reach of calculation. The discovery that blood circulates in animals and sap in plants has changed our understanding of nature. The air pump has given bodies a new way of existing. Telescopes have brought objects closer to our eyes. Finally, what Newton discovered about light is as bold as anything human curiosity could have hoped for, even after so many new discoveries.
Until Antonio de Dominis, the rainbow had seemed an inexplicable miracle. This philosopher guessed that it was a necessary result of rain and sunshine. Descartes made his name immortal by explaining this perfectly natural event mathematically. He calculated how light is reflected in raindrops, and his insight seemed almost divine at the time.
But what would he have said if someone had told him that he was wrong about the nature of light? That he had no reason to insist it was made of tiny spheres? That it was false to say that this matter, spread throughout the universe, merely waits to be set in motion by a push from the sun, like a long stick that moves at one end when the other is pushed? That light is in fact sent out by the sun and takes nearly seven minutes to travel from the sun to the earth, while a cannonball, even if it kept up its speed, could travel that distance only in twenty-five years?
How surprised would he have been to hear someone say: “It is false that light is reflected directly by bouncing off the solid parts of a body. It is false that bodies are transparent when they have wide pores. A man will come who will prove these unlikely claims and take apart a single ray of light more skillfully than the best practitioner dissects the human body!”
He understood light so well that he determined the limits of what we can do to make it stronger and help our eyes with telescopes.
Descartes had a noble confidence, quite forgivable in light of his excitement at the beginnings of a field that he had almost discovered himself. He hoped that telescopes would let us see objects among the stars as small as those we can distinguish on earth.
Newton showed that telescopes cannot be improved any further because of refraction itself. Refraction brings objects closer to us but spreads the basic rays too far apart. He calculated how much the red and blue rays spread apart in these lenses. He even proved things whose existence no one had suspected. He examined the irregularities caused by the shape of the glass and those caused by the different refrangibility of the rays. He found that if the telescope's objective lens is convex on one side and flat on the other, with the flat side facing the object, the defect caused by the lens's shape and position is five thousand times smaller than the defect caused by refrangibility. So it is not the shape of the lenses that prevents further improvement of telescopes. The problem lies in the nature of light itself.
That is why he invented a telescope that shows objects by reflection rather than refraction.
He was still little known in Europe when he made this discovery. I saw a little book written around that time that calls him a maker of eyeglasses when it mentions Newton's telescope: “A certain English craftsman named Newton.” Later generations have more than made up for the slight.