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
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