OPTICAL TREATISE
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HYPOTHESES
[217] 1. Every action is motion in the agent, just as every passion is motion in the patient. By the name agent, I mean a body, the motion of which produces an effect in another body. By the name patient, I mean a body in which a motion is set up by another body. For example, when ‘one nail drives out another’ (as the saying goes), [n.1] the motion of the nail which is doing the driving is its action, and the motion of the one being driven is its passion. Similarly, when burning coal warms a person, neither the person nor the coal leaves its place, and hence neither of them moves. However, there is some matter or subtle body in the coal which does move, and it sets up a motion in the medium between it and the person, and a certain motion is then generated in the internal parts of the person, although they are standing still. This motion in the internal parts of the person is heat. To move or to be warmed in this way is to be passive. The motion which is in the parts of the lighted coal is its action, or warming; and to move in this way is to warm.
2. Seeing is a passion produced in the person who sees by the action of a luminous or illuminated object.
3. In seeing, neither the object nor any part of it passes from where it is to the eye. In order for motion to give rise to motion at any distance whatever, it is not necessary for the body which gives rise to the motion to cross the whole space through [218] which the motion is propagated. It is enough that it should move so little that its motion is insensible, and give an impulse to what is situated next to it. This next body stays where it is, but having been impelled, it also impels what is next to it. In this way, the motion will be propagated as far as you like.
4. Every luminous body [n.2] can be seen by any number of observers from any direction at the same time.
5. I call the medium more ‘rarefied’ the less it resists the transfer of motion, and more ‘dense’ the more it resists it. I assume that air is rarer than water, water than glass, and glass than crystal.
PROPOSITION 1
Every luminous body expands, and swells into a larger mass; and then it contracts again. Thus it has a perpetual systole and diastole. [n.3]
Since (by hypothesis number 4) every luminous body is seen from every direction at the same time; and seeing occurs (by hypothesis number 2) as the result of the action of the luminous body; and (by hypothesis number 1) every action consists of a motion in the agent; it follows that in a luminous body there is a motion in all directions at the same time. But when luminous bodies are seen, they are not scattered into parts which go into the eyes of people seeing them from every direction, otherwise they would disappear. The only alternative is that the parts of the luminous body, which have been proved to move in every direction at the same time, revert to their previous positions. But this is the same as to say that the whole luminous body expands and contracts alternately, or has a perpetual systole and diastole. QED.
So we also see that what we observe in every luminous body, and call ‘scintillation’, is nothing other than this systole and diastole.
PROPOSITION 2
Motion is propagated from a luminous body to the eye by means of a continuous pushing back of a contiguous part of the medium.
We have assumed (hypothesis number 3) that neither the object nor any part of it whatever travels to the eye when it is seen. But it is impossible to think of any way in which motion can be propagated over a distance, other than the one I have proposed. So it follows that this is the way it happens.
PROPOSITION 3
To consider how light occurs, and what it is.
Let there be a luminous body, the sun, with its centre at A, and a radius AB. It is surrounded by a concentric orb, the thickness of which is BC.
By ‘orb’, I mean a solid bounded by two concentric spherical surfaces. Again, let there be another concentric orb CD surrounding the orb BC, and yet another orb DE surrounding the orb CD. In the same way, any number of orbs can be constructed, each having the same volume. So since the outer circumferences are always bigger than the inner ones, the thickness of the inner orbs will be greater than that of the outer ones. Therefore BC is greater than CD, and CD is greater than DE. Now since the sun swells and expands into a larger mass through the first orb, let us suppose that when the sun is in diastole (i.e. expanding), it equals the whole sphere with a radius of AC. Then necessarily the part of the medium which was in the orb BC will move outwards into the next equal volume of space, namely the orb CD — and at one and the same time. For at the very instant when [220] motion from B towards C begins, there must necessarily begin a motion from C towards D, and from D towards E, and further outwards from E. Consequently, if the eye is placed at any distance from the sun, let us suppose at E, at the very instant when the sun begins to expand at B, at the same instant the eye receives the motion at E. From there the motion is propagated to the retina, and from there to the brain through the optic nerve because of the conation of the retina. [n.4] This happens at the same instant as the motion begins at B. Furthermore, as in everything else which is acted upon, there is a reaction in the brain, so that a motion is propagated back again from the brain through the optic nerve into the retina, and from there towards the sun along the same line as the previous motion travelled from the sun to the retina. And as I have already proved of the motion from the sun to the eye, this whole process will have taken place in an instant. So it is obvious that in all cases of seeing, a motion is propagated from the luminous body to the eye and to the brain, and then back again to places outside the eyes — and all in an instant. It is also obvious that the motion which is thus propagated from the luminous body is weaker at a great distance than at a short one. For since BC is bigger than CD, and CD than DE, but the time of propagation from B to C is the same as from C to D, or from D to E, the motion propagated is quicker in BC than in CD, and in CD than in DE, and so on.
So far we have considered what the motion from the luminous body is like. Now we must consider why and when such motion is called ‘light’.
Firstly, if there were no seeing, there would be nothing which we would call ‘light’; for the congenitally blind have no understanding of talk of light and colours. Consequently, the motion is not called ‘light’ before seeing takes place — that is, before it reaches the brain. Secondly, the motion in the brain which we call ‘light’ is not sensed by us in the brain itself, [221] but outside, and before our eyes. Consequently, we do not call the motion from the luminous body ‘light’, until the reaction propagates it back from the brain through the optic nerve and the eyes to the medium between the eye and the luminous body. Therefore light is an apparition before the eyes of the motion which is propagated by the diastole (or expansion) of the luminous object to the brain, and then back again through the eyes to the medium. So light is a phantasm of the luminous body, or an image of it conceived in the brain. This is also confirmed by experience, in that a light appears before the eyes whenever the brain is made to vibrate in such a way as to generate an outward motion through the optic nerve (for example, when the eye is hit). From what has been said, I shall draw a brief corollary.
COROLLARY
Light is a phantasm from a luminous body. The same is true of colours, which are turbulent light.
Light is propagated to any distance whatever in an instant.
The further light is propagated, the weaker it becomes.
NOTE
The great difficulty of forming a conception of light has racked the wits of both ancient and modern philosophers. I hope they will be convinced if they consider that hardly anything (in fact nothing at all) is conceived by us clearly and distinctly except through motion and shapes. Anyone who studies shapes diligently, and understands the compositions of motions, [n.5] will admit that, in the whole of philosophy, nothing is easier, more amenable to demonstration, or more suited to human wit. . . . .
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