Download Since light can be generated only in matter, where there is more

An Early Passage on Refraction 1
Since light can be generated only in matter, where there is more matter light is generated
more easily all things being equal; therefore it will penetrate more easily (facilius) a
denser medium than a rarer medium. Whence it happens that refraction occurs in the
latter case away from the perpendicular and in the former towards the perpendicular;
moreover the greatest refraction of all would be along the perpendicular itself, if the
medium were as dense as possible (densissiumum); from which [medium] the ray leaving
again would leave through the same angle. Let abcd be a medium as dense as possible,
ray ef will cross perpendicularly through fg to gh, so that bfe and cgh are equal angles.
Reflection, however, is nothing other than the production (production) of light from an
opaque surface in a changed direction since it cannot go in a straight line. For example,
the surface afb produces a reflected ray fi, which the surface cgd would have produced
along the straight line gh. The place of the image is along a straight line from the eye to
the first point of the reflection or refraction produced. In the place, however, where the
point of reflection or refraction occurs, this is apparent only from the position of other
points because the distance of an object is not noticed otherwise. Indeed, it could be said
that [the place of the image is along the perpendicular from the object; for that one place
happens per accidens in certain things, and not from the fact that the concursus of the
perpendicular occurs (id enim unum fit per accidens in quibusdam, and non ex eo quod fit
concursus perpendicularis).
1
This is a selection taken from Descartes’ early Cogitationes Privatae of 1619-1621.
The Latin text appears in Oeuvres de Descartes, Charles Adam and Paul Tannery, (eds.)
1904. Paris: J. Vrin., volume X, pages 242-3. Thanks to Paul Hoffman and Benjamin
King for helpful suggestions concerning the present translation.