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On the subject of light, the entry LUMIÈRE (LIGHT) in the DUF-1690 says:

It is a very subtle, prompt, and uncluttered body that causes clarity, that illuminates, that gives color to all things, that shakes the eyes, and makes objects visible. Philosophers distinguish between primitive, or radical light, and second or derivative light. Primitive or radical light is that which is in the luminous body, and which consists of a certain movement of its parts, which makes them capable of pushing the subtle matter around, which fills the pores of transparent bodies. And the second or derived light is nothing else than the inclination to move, or the tendency of this subtle matter to move away in a straight line from the center of the luminous body.

Against the Cartesian conception expressed above, borrowed from Jacques Rohault, the author opposes the atomist conception of Pierre Gassendi, which is also that of Newton, for whom light consists of the flow of an infinite number of light corpuscles “spreading with incredible speed on all sides”. René Descartes and Christiaan Huygens rejected the corpuscular approach to light, because how can such a considerable quantity of corpuscles be emitted by luminous bodies without being exhausted, and, on the other hand, how is it that inflamed corpuscles do not heat up the optic nerve at the same time as they illuminate it?

Mr. Huygens designed a long series of globules that form like little sticks, with one end touching the sun and the other touching the back of the eye. After this, it must follow, that at the same instant that the Sun presses the end that is contiguous to it, the one that presses on the eye is also pressed. Thus the light reaches us from the luminous body by some movement imprinted on the subtle matter in between.

DUF-1727 presents some additional elements, mentioning the equivocal character of light, which can be taken as “the particular feeling that the soul receives by the impression that luminous bodies make on the eyes”, or to designate “what is in these bodies by which they cause this particular feeling in the soul.” The same questioning is expressed for heat, as we have seen. The works of Nicolas Malebranche treating light by analogy with sound are mentioned. The time it takes light to pass from the Sun to the Earth, estimated at 11 minutes by Ole Christensen Roemer, thanks to the measured time lag of the moments when Jupiter eclipses some of its satellites, is indicated, and some considerations on the propagation of light and its colors are stated. The Encyclopédie takes up exactly the definition of the DUF, with the ambiguity between purely sensory effect and physical phenomenon. The Cartesian doctrine of the matter of the first element being agitated, which presses in all directions the small globules of the second element, compared to hard spheres that touch each other and instantaneously transmit the action of light to our eyes, is briefly described for criticism. The globules must be elastic, and not hard, for at least two reasons: first of all, the light is not transmitted instantaneously, as thought Descartes, then the light is reflected, which is not compatible with perfectly hard globules. Malebranche, for whom the parts of the luminous body in fast movement excite vibrations of pressure in the subtle matter which is between him and our eye, the amplitude and the frequency of these vibrations conditioning, respectively, the intensity and the color of the transmitted light, replaces the hard globules by swirls, while preserving the general design of Descartes and Huygens of a propagation of the light in the form of wave on a substrate of subtle matter. Huygens’ theory makes it possible to explain the refraction and the reflection, but does not account simply for the propagation of the light in a straight line, which, like sound, should propagate in all directions according to the undulatory assumption. Newton explained the straight-line propagation by the corpuscular hypothesis, “the action by which the body produces in us the sensation of clarity, consisting not in an effort to move, but in the real movement of these particles which move away from all sides of the luminous body in a straight line, and with an almost incredible speed,” Any obstacle in the path of a wave curves the train, and if the light were a wave, “the shadow would continuously bend it in its path”. As much as sound can follow a curved path, “we have never seen light move in a curved line; light rays are therefore small corpuscles that run with great speed from the luminous body.”

Then, the author asks the question of the link between heat and light. He says that he cannot answer this question on the basis of experience for the simple reason that heat and light can present very small variations, below our threshold of perception, which we are not able to evaluate. Then, he gives Newton’s view of the matter–heat–light relationship:

Mr. Newton observes that bodies and rays of light act continuously on each other; bodies on rays of light, by throwing, reflecting, and refracting them; and rays of light on bodies, by heating them, and giving their parts a vibrating motion of which mainly heat consists: for he also notices that all fixed bodies, when they have been heated beyond a certain degree, become luminous, a quality which they seem to owe to the vibrating movement of their parts; and finally, that all bodies which abound in earthly and sulfurous parts, give off light if they are sufficiently agitated in any way. Thus, the sea becomes luminous in a storm; quicksilver, when shaken in a vacuum; cats and horses, when rubbed in the dark; wood, fish, and meat, when rotten.

The author ends his entry by admitting his inability to choose between the wave hypothesis and the corpuscular hypothesis, of which “neither are demonstrated; and the wisest answer to the question of matter and the propagation of light would perhaps be to say that we don’t know.” He concludes by expressing the essential laws of optics, catoptrics and dioptrics: “With these simple propositions, the theory of light becomes a purely geometrical science, and its properties are demonstrated without knowing what it consists of or how it propagates.”

For the author of the Lexicon entry LIGHT, the origin of the light must undoubtedly be attributed to movement. It is not about the movement of a subtle matter, like the first element of Descartes, but about the movement of the ordinary matter likely to give place to a light emission. The author refers to Hooke, who judges that movement, to produce light, must satisfy two conditions: (i) it must be extremely fast, like that of fermentation and putrefaction, which makes brine and rotten wood shine and (ii) it must be vibratory, with vibrations of an extremely short period, as well as those of rubbed diamonds which become shiny. These phenomena, which today would be called chemiluminescence and triboluminescence, are characteristic of phosphorus, which we have previously described. There is no question here of fire and heat as sources of light, the latter being found in movement, from which of course it can result some heat, according to Boyle’s theory that we have exposed, and some fire, for example, by friction. The author then states elements of Newton’s work on light (refraction, colors), and notes that Newton, because of the straight-line propagation of light, considers that this propagation cannot consist only in the Cartesian principle of action.

An essential aspect of the article, largely absent from the DUF and the Encyclopédie, is the long development devoted to the demonstration made by William Molyneux that light is a “body”, and therefore matter, in the sense of physical matter, and not subtle, as considered by most French scientists of the time. Molyneux, in his Dioptrique, identifies three properties of light that show its material nature. First of all, the phenomenon of refraction, which shows that light passing through diaphanous bodies is resistant to it. What, if not a body, can be resisted by a medium?

This resistance to the passage of light through different diaphanous bodies can be understood as resulting from the fact that the medium prevents light from diffusing and distributing itself in all parts of the medium, and therefore the medium can be said to be less illuminable: because, by its nature, light tends to diffuse. And, conversely, the more the light affects the parts of the medium that it illuminates equally and uniformly, the greater the number of particles of the illuminated medium to which it transmits its energy, the more the medium can be said to be illuminable, less resistant to the progression of light. Hence the fact that, the more solid and small the affected parts of the medium are, the less space they admit between them for any other heterogeneous matter, the more important is the illumination of the medium. And it is certain that resistance must result from the contact of two bodies, and that contact, whether active or passive, is the property of the bodies.

Thus, the interaction between light and the medium is conceived in terms of mechanical action and reaction between bodies, the resistance of the medium being all the weaker as it is cohesive and admits fewer impurities in its pores, potentially slowing down the light by preventing it from diffusing. The second property confirming that light is a body, and a “body moved and projected forward”, is that its passage from one place to another is not instantaneous, but takes a certain amount of time, its movement being the fastest of all. The author cites Roemer’s observation of the eclipses of the satellites of Jupiter, which allowed him to estimate the speed of light. A third piece of evidence put forward by Molyneux is that “light cannot, by any technical or artificial process, be increased or decreased”. One cannot, for example, enlarge the light of the Sun or of a candle, any more than 1 cubic inch of gold can be enlarged:

For every time we see the light increase, it is at the expense of some other part of the environment that has lost light, or that light, which naturally should have spread to other parts, has been brought to the brighter place. Thus, for example, in a magnifying glass that concentrates the Sun’s light at its focal point, or point of combustion; we must first consider that the image of the Sun is projected at the focal point on a separate base from that of the glass. And secondly, we can observe all around the bright spot of the image of the Sun the marked shadow that the full width of the magnifying glass casts on it: for all the rays of the Sun, which would have fallen on this wide space of shadow, are now gathered and concentrated in the bright spot, producing vigorous light and violent heat.

An objection can be made that the light is increased by reflection, without depriving any other place of the light it would otherwise have received. It is easy to see that this objection does not hold. If we imagine a candle placed in a room facing a small opening to the outside, half of the light from the flame illuminates the room, while the other half illuminates the outside space. If we now place in front of the opening a mirror with its reflective side facing inwards, the light that previously illuminated the outside of the room illuminates the inside of the room, to the detriment of the outside space which is no longer illuminated. We thus see that the mirror subtracts the light in the middle behind it, and the contradiction raised falls.

The content of the Lexicon entry thus significantly differs from the contents of the articles of the DUF and the Encyclopédie, in that it focuses, with the exception of the passages devoted to the laws of propagation of light, on the question of the character of light as a body, and therefore as matter. The Lexicon entry makes almost no reference to heat, and to the possible relations between heat and light, a subject of concern to French scientists, and cites as examples, in support of the idea that light is born of movement, only those of phosphorus whose light is not accompanied by fire or heat. Light is presented as a matter in its own right, in mechanical interaction with its environment.