Contact Forces

I have then shown:

• how one may conceive Light to spread successively, by spherical waves
• how this spreading is accomplished with as great a velocity as that which experiments and celestial observations demand.

The particles are supposed to be in continual movement. There are many reasons for this.

• But the successive propagation of the waves cannot be hindered by this because the propagation consists nowise in the transport of those particles
• Instead, it is merely in a small agitation which they cannot help communicating to those surrounding, notwithstanding any movement which may act on them causing them to be changing positions amongst themselves.

What is the origin of these waves? How do they spread?

Each little region of a luminous body, such as the Sun, a candle, or a burning coal, generates its own waves from its center.

A flame has the points A, B, C.

• Concentric circles around each of these points represent the waves which come from them.

The same is true for every point of the surface and of the part within the flame.

But the percussions at the centres of these waves have no regular succession.

• It must not be supposed that the waves follow one another at equal distances.

If the distances marked in the shape appear to be such, it is rather to mark the progression of one-and-the-same wave at equal intervals of time than to represent several of them issuing from one and the same centre.

This prodigious quantity of waves traverse one another without confusion and without effacing one another.

This is because one-and-the-same particle of matter can serve for many waves coming from different sides or even from contrary directions, not only if it is struck by blows which follow one another closely but even for those which act on it at the same instant.

It can do so because the spreading of the movement is successive.

This may be proven by the row of hard, equal spheres, spoken of above.

• Two similar spheres A and D are pushed from 2 opposite sides at the same time as shown below.
• Each sphere will rebound with the same velocity which it had after striking.
• Yet the whole row will remain in its place, even if the movement has passed along its whole length twice over.

If these contrary movements happen to meet at the middle sphere B or at C, that sphere will yield.

• It will act as a spring at both sides and simultaneously transmit these 2 movements.

It is incredible that the undulations produced by such small movements and corpuscles would spread to immense distances. An example is from the stars to us.

The force of these waves must grow feeble as they move away from their origin.

• The action of each one will be incapable of being seen by us.

From a great distance, an infinitude of waves issues from the luminous body at different points. These unite together in such a way that they sensibly compose one single wave only, which, consequently, ought to have enough force to make itself felt.

Thus this infinite number of waves which originate at the same instant from all points of a fixed star make practically only one single wave which has enough force to impress on our eyes.

From each luminous point, there may come many thousands of waves in the smallest time, by the frequent percussion of the corpuscles which strike the Ether at these points. This contributes to making their action more sensible.

The Emanation of These Waves

Each particle of matter in which a wave spreads does not communicate its motion only to the next particle ahead of it in a straight line from the luminous point.

It also imparts some of it to all the others which touch it and which oppose themselves to its movement.

So around each particle is a wave with the centre in that particle.

If DCF is a wave emanating from the center of luminous point A, the particle B, one of those comprised within the sphere DCF, will have made its particular or partial wave KCL, which will touch the wave DCF at C at the same moment that the principal wave emanating from the point A has arrived at DCF.

Only the region C of the wave KCL will touch the wave DCF, that which is in the straight line drawn through AB.

Similarly, the other particles of the sphere DCF, such as bb, dd, etc. will each make its own wave.

But each of these waves can be infinitely feeble only as compared with the wave DCF, to the composition of which all the others contribute by the part of their surface which is most distant from the centre A.

The wave DCF is determined by the distance attained in a certain space of time by the movement which started from the point A.

There is no movement beyond this wave. Though there will be in the space which it encloses, namely in parts of the particular waves, those parts which do not touch the sphere DCF.

All the properties of Light, and everything pertaining to its reflexion and its refraction, can be explained by this.

The following had tried to prove the effects of reflexion and refraction by these waves:

• Hooke in his Micrographia
• Father Pardies

They did not know this principle.

The Properties of Light

Each portion of a wave should spread in such a way that its extremities lie always between the same straight lines drawn from the luminous point.

Thus, the portion BG of the wave has the luminous point A as its centre. It will spread into the arc CE bounded by the straight lines ABC, AGE.

The waves produced by the particles within the space CAE spread also outside this space.

But they yet do not concur at the same instant to compose a wave which terminates the movement, as they do precisely at the circumference CE, which is their common tangent.

This is why light spreads only by straight lines.

If there were an opening BG, limited by opaque bodies BH, GI, the wave of light which issues from the point A will always be terminated by the straight lines AC, AE, as has just been shown.

The parts of the partial waves which spread outside the space ACE are too feeble to produce light there.

However small the opening BG is, the light will always pass through straight lines.

• This opening is always large enough to contain many particles of the ethereal matter which are of an inconceivable smallness.
• Each little portion of the wave necessarily advances following the straight line which comes from the luminous point.

Thus we may take the rays of light as if they were straight lines.

It appears, moreover, by what has been remarked touching the feebleness of the particular waves, that it is not needful that all the particles of the Ether should be equal amongst themselves, though equality is more apt for the propagation of the movement.

Inequality will cause a particle by pushing against another larger one to strive to recoil with a part of its movement. But it will thereby merely generate backwards towards the luminous point some partial waves incapable of causing light, and not a wave compounded of many as CE was.

One of the most marvellous property of waves of light is that when some of them come from different or even from opposing sides, they produce their effect across one another without any hindrance.

• Spectators may view different objects at the same time through the same opening
• Two persons can at the same time see one another’s eyes.

Now according to the explanation which has been given of the action of light, how the waves do not destroy nor interrupt one another when they cross one another, these effects which I have just mentioned are easily conceived.

But in my judgement they are not at all easy to explain according to the views of

Descartes says that Light consists of a continuous pressure merely tending to movement.

For this pressure not being able to act from two opposite sides at the same time, against bodies which have no inclination to approach one another, it is impossible so to understand what I have been saying about 2 persons mutually seeing one another’s eyes, or how two torches can illuminate one another.