Treatise on Light

...things were shrouded, and to offer hope that they might be explained by intelligible reasons. On the other hand, I am also surprised how those very authors have often tried to pass off arguments that are hardly evident as being certain and demonstrative; for I find that no one has yet provided a probable explanation for these primary and notable phenomena of light, namely: why it only extends along straight lines, and how visual rays, coming from an infinite number of different places, cross one another without interfering with each other at all.

In this book, I shall therefore attempt, using principles accepted in today’s philosophy Huygens refers to the "mechanical philosophy" prevalent in the 17th century, which sought to explain natural phenomena through the motion and collision of physical particles., to provide clearer and more plausible reasons: first, for these properties of light traveling in straight lines; second, for light that is reflected when meeting other bodies. Then, I will explain the characteristics of rays that are said to undergo refraction when passing through diaphanous bodiesMaterials that are transparent or allow light to pass through, such as glass or water. of various kinds; where I will also treat the effects of the refraction of air caused by the differing densities of the atmosphere.

Next, I will examine the causes of the strange refraction of a certain crystal brought from IcelandKnown today as "Iceland Spar" (a form of calcite), this crystal exhibits double refraction, a phenomenon where a single ray of light enters the crystal and splits into two.. And finally, I will treat the various shapes of transparent and reflecting bodies by which rays are gathered at a point or diverted in different ways. There, it will be seen with what ease our new Theory allows us to find not only the ellipses, hyperboles, and other curved lines that Mr. DescartesRené Descartes (1596–1650), whose work Dioptrics was the standard text on the subject before Huygens. subtly invented for this purpose, but also those that must form the surface of a lens when the other surface is already given as spherical, flat, or of any other shape.

One cannot doubt that light consists of the motion of a certain type of matter. For whether one considers its produc-