Treatise on Light

CHAPTER I.. On Rays Propagated in Straight Lines.

On Rays Propagated in Straight Lines.

That Light is produced by a certain movement.

That no substance passes from the luminous object to the eyes.

That Light spreads spherically, almost as Sound does.

Whether Light takes time to spread.

Experience seeming to prove that it passes instantaneously.

Experience proving that it takes time.

How much its speed is greater than that of Sound.

In what the emission of Light differs from that of Sound.

That it is not the same medium which serves for Light and Sound.

How Sound is propagated.

How Light is propagated.

Detailed Remarks on the propagation of Light.

Why Rays are propagated only in straight lines.

How Light coming in different directions can cross itself.

CHAPTER II.. On Reflexion.

On Reflexion.

Demonstration of equality of angles of incidence and reflexion.

Why the incident and reflected rays are in the same plane perpendicular to the reflecting surface.

That it is not needful for the reflecting surface to be perfectly flat to attain equality of the angles of incidence and reflexion.

CHAPTER III.. On Refraction.

On Refraction.

That bodies may be transparent without any substance passing through them.

Proof that the ethereal matter passes through transparent bodies.

How this matter passing through can render them transparent.

That the most solid bodies in appearance are of a very loose texture.

That Light spreads more slowly in water and in glass than in air.

Third hypothesis to explain transparency, and the retardation which Light suffers.

On that which makes bodies opaque.

Demonstration why Refraction obeys the known proportion of Sines.

Why the incident and refracted Rays produce one another reciprocally.

Why Reflexion within a triangular glass prism is suddenly augmented when the Light can no longer penetrate.

That bodies which cause greater Refraction also cause stronger Reflexion.

Demonstration of the Theorem of Mr. Fermat.

CHAPTER IV.. On the Refraction of the Air.

On the Refraction of the Air.

That the emanations of Light in the air are not spherical.

How consequently some objects appear higher than they are.

How the Sun may appear on the Horizon before he has risen.

That the rays of light become curved in the Air of the Atmosphere, and what effects this produces.

CHAPTER V.. On the Strange Refraction of Iceland Crystal.

On the Strange Refraction of Iceland Crystal.

That this Crystal grows also in other countries.

Who first-wrote about it.

Description of Iceland Crystal; its substance, shape, and properties.

That it has two different Refractions.

That the ray perpendicular to the surface suffers refraction, and that some rays inclined to the surface pass without suffering refraction.

Observation of the refractions in this Crystal.

That there is a Regular and an Irregular Refraction.

The way of measuring the two Refractions of Iceland Crystal.

Remarkable properties of the Irregular Refraction.

Hypothesis to explain the double Refraction.

That Rock Crystal has also a double Refraction.

Hypothesis of emanations of Light, within Iceland Crystal, of spheroidal form, for the Irregular Refraction.

How a perpendicular ray can suffer Refraction.

How the position and form of the spheroidal emanations in this Crystal can be defined.

Explanation of the Irregular Refraction by these spheroidal emanations.

Easy way to find the Irregular Refraction of each incident ray.

Demonstration of the oblique ray which traverses the Crystal without being refracted.

Other irregularities of Refraction explained.

That an object placed beneath the Crystal appears double, in two images of different heights.

Why the apparent heights of one of the images change on changing the position of the eyes above the Crystal.

Of the different sections of this Crystal which produce yet other refractions, and confirm all this Theory.

Particular way of polishing the surfaces after it has been cut.

Surprising phenomenon touching the rays which pass through two separated pieces; the cause of which is not explained.

Probable conjecture on the internal composition of Iceland Crystal, and of what figure its particles are.

Tests to confirm this conjecture.

Calculations which have been supposed in this Chapter.

CHAPTER VI.

On the Figures of transparent bodies which serve for Refraction and for Reflexion.

General and easy rule to find these Figures.

Invention of the Ovals of Mr. Des Cartes for Dioptrics.

How he was able to find these Lines.

Way of finding the surface of a glass for perfect refraction, when the other surface is given.

Remark on what happens to rays refracted at a spherical surface.

Remark on the curved line which is formed by reflexion in a spherical concave mirror.

CHAPTER I. On Rays Propagated in Straight Lines

ON RAYS PROPAGATED IN STRAIGHT LINES

As happens in all the sciences in which Geometry is applied to matter, the demonstrations concerning Optics are founded on truths drawn from experience. Such are that the rays of light are propagated in straight lines; that the angles of reflexion and of incidence are equal; and that in refraction the ray is bent according to the law of sines, now so well known, and which is no less certain than the preceding laws.

The majority of those who have written touching the various parts of Optics have contented themselves with presuming these truths. But some, more inquiring, have desired to investigate the origin and the causes, considering these to be in themselves wonderful effects of Nature. In which they advanced some ingenious things, but not however such that the most intelligent folk do not wish for better and more satisfactory explanations. Wherefore I here desire to propound what I have meditated on the subject, so as to contribute as much as I can to the explanation of this department of Natural Science, which, not without reason, is reputed to be one of its most difficult parts. I recognize myself to be much indebted to those who were the first to begin to dissipate the strange obscurity in which these things were enveloped, and to give us hope that they might be explained by intelligible reasoning. But, on the other hand I am astonished also that even here these have often been willing to offer, as assured and demonstrative, reasonings which were far from conclusive. For I do not find that any one has yet given a probable explanation of the first and most notable phenomena of light, namely why it is not propagated except in straight lines, and how visible rays, coming from an infinitude of diverse places, cross one another without hindering one another in any way.

I shall therefore essay in this book, to give, in accordance with the principles accepted in the Philosophy of the present day, some clearer and more probable reasons, firstly of these properties of light propagated rectilinearly; secondly of light which is reflected on meeting other bodies. Then I shall explain the phenomena of those rays which are said to suffer refraction on passing through transparent bodies of different sorts; and in this part I shall also explain the effects of the refraction of the air by the different densities of the Atmosphere.

Thereafter I shall examine the causes of the strange refraction of a certain kind of Crystal which is brought from Iceland. And finally I shall treat of the various shapes of transparent and reflecting bodies by which rays are collected at a point or are turned aside in various ways. From this it will be seen with what facility, following our new Theory, we find not only the Ellipses, Hyperbolas, and other curves which Mr. Des Cartes has ingeniously invented for this purpose; but also those which the surface of a glass lens ought to possess when its other surface is given as spherical or plane, or of any other figure that may be.

It is inconceivable to doubt that light consists in the motion of some sort of matter. For whether one considers its production, one sees that here upon the Earth it is chiefly engendered by fire and flame which contain without doubt bodies that are in rapid motion, since they dissolve and melt many other bodies, even the most solid; or whether one considers its effects, one sees that when light is collected, as by concave mirrors, it has the property of burning as a fire does, that is to say it disunites the particles of bodies. This is assuredly the mark of motion, at least in the true Philosophy, in which one conceives the causes of all natural effects in terms of mechanical motions. This, in my opinion, we must necessarily do, or else renounce all hopes of ever comprehending anything in Physics.

And as, according to this Philosophy, one holds as certain that the sensation of sight is excited only by the impression of some movement of a kind of matter which acts on the nerves at the back of our eyes, there is here yet one reason more for believing that light consists in a movement of the matter which exists between us and the luminous body.

Further, when one considers the extreme speed with which light spreads on every side, and how, when it comes from different regions, even from those directly opposite, the rays traverse one another without hindrance, one may well understand that when we see a luminous object, it cannot be by any transport of matter coming to us from this object, in the way in which a shot or an arrow traverses the air; for assuredly that would too greatly impugn these two properties of light, especially the second of them. It is then in some other way that light spreads; and that which can lead us to comprehend it is the knowledge which we have of the spreading of Sound in the air.

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