CHAPTER I

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