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
Theory, we find not only the Ellipses, Hyperbolas, and other curves
which Mr. Des Cartes has ingeniously invented for this purpose; but
also those which the surface of a glass lens ought to possess when its
other surface is given as spherical or plane, or of any other figure
that may be.

It is inconceivable to doubt that light consists in the motion of some
sort of matter. For whether one considers its production, one sees
that here upon the Earth it is chiefly engendered by fire and flame
which contain without doubt bodies that are in rapid motion, since
they dissolve and melt many other bodies, even the most solid; or
whether one considers its effects, one sees that when light is
collected, as by concave mirrors, it has the property of burning as a
fire does, that is to say it disunites the particles of