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.
1.6. Ether
The entry ETHER in both the DUF and the Encyclopédie sheds complementary light. Ether is, according to the DUF-1690, “that pure substance which is above the atmosphere, which fills all the sky where the stars take their course.” The 1727 edition offers further information on the rarity of ether in relation to air, and the existence of different ethers of varying degrees of rarity, “making ether a suitable means of transmitting light and the influences of the most distant stars.” The entry in the Encyclopédie uses a definition of ether quite similar to that in the DUF. Ether, whose necessity of existence is invoked to prevent “the majority of the universe from being entirely empty”, either fills only the space between the celestial bodies, above the atmosphere, or is “of such a subtle nature, that it penetrates the air and other bodies, and occupies their pores and intervals.” Some believed that ether
