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T R E A T I S E

O N L I G H T.

C H A P. I.

ON RAYS TRAVELING IN STRAIGHT LINES.

The proofs concerning OpticsThe branch of physics that studies the behavior and properties of light., as occurs in all the sciences where Geometry is applied to matter, are founded on truths drawn from experience; such as that rays of light extend in a straight line; that the angles of reflection and incidence are equal: and that in refractions the ray is bent according to the Rule of SinesNow commonly known as Snell’s Law, this formula describes how light bends when passing from one medium (like air) into another (like glass or water)., which is now so well known, and which is no less certain than the preceding ones.

Most of those who have written concerning the various parts of Optics have been content to take these truths for granted. But some more inquisitive minds have sought to investigate their origin and causes, considering the truths themselves as wonderful effects of Nature. While they have put forward ingenious ideas, they are not yet such that the most intelligent readers do not wish for explanations that satisfy them more; I wish to propose here what I have meditated on this subject, to contribute as much as I can to the clarification of this part of Natural Science, which is considered, not without reason, to be one of its most difficult branches. I recognize that I am much indebted to those who first began to dispel the strange obscurity in which these thi-