Particular way of polishing the surfaces after it has been cut.

Surprising phenomenon touching the rays which pass through two
separated pieces; the cause of which is not explained.

Probable conjecture on the internal composition of Iceland Crystal,
and of what figure its particles are.

Tests to confirm this conjecture.

Calculations which have been supposed in this Chapter.

CHAPTER VI.
On the Figures of transparent bodies which serve for Refraction and
for Reflexion.

General and easy rule to find these Figures.

Invention of the Ovals of Mr. Des Cartes for Dioptrics.

How he was able to find these Lines.

Way of finding the surface of a glass for perfect refraction, when
the other surface is given.

Remark on what happens to rays refracted at a spherical surface.

Remark on the curved line which is formed by reflexion in a spherical
concave mirror.