Chapter 1

Rays Propagated In Straight Lines

by Christiaan Huygens Icon

As happens in all the sciences in which Geometry is applied to matter, the demonstrations concerning Optics are founded on truths drawn from experience.

Examples are:

  • the rays of light propagate in straight lines
  • the angles of reflection and of incidence are equal
  • in refraction, the ray is bent according to the law of sines

Most of those who have written about Optics are content with presuming these truths.

But some are more inquiring.

  • These investigated the origin and the causes, considering these to be in themselves wonderful effects of Nature.
  • They advanced some ingenious things that the most intelligent people do not wish for better explanations.

I have meditated on the subject so as to contribute as much as I can to explain this department of Natural Science, which is reasonably 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 I am also 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 give in this book some clearer and more probable reasons for:

  1. These properties of light propagated rectilinearly
  2. These properties of light 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
  • the effects of the refraction of the air by the different densities of the atmosphere
  • the causes of the strange refraction of a certain kind of the Iceland Crystal
  • 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, our new Theory will make it easier for us to find:

  • the Ellipses, Hyperbolas, and other curves which Descartes has ingeniously invented for this purpose
  • those which the surface of a glass lens should possess when its other surface is given as spherical, plane, or any other shape

We may doubt that light consists in the motion of some sort of [aethereal] matter because of the cause and effect of light.

Light on Earth is chiefly produced by fire and flame.

  • These contain bodies that are in rapid motion
  • This motion lets them dissolve and melt many other bodies, even the most solid

Light, as an effect, can burn just as a fire does when it is collected by concave mirrors.

  • It can disunite the particles of bodies.

This is assuredly the mark of motion, at least in true Philosophy.

  • The true Philosophy says describes the causes of all natural effects in terms of mechanical motions.

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