xion.

CHAPTER III.
On Refraction.

That bodies may be transparent without any substance passing through
them.

Proof that the ethereal matter passes through transparent bodies.

How this matter passing through can render them transparent.

That the most solid bodies in appearance are of a very loose texture.

That Light spreads more slowly in water and in glass than in air.

Third hypothesis to explain transparency, and the retardation which
Light suffers.

On that which makes bodies opaque.

Demonstration why Refraction obeys the known proportion of Sines.

Why the incident and refracted Rays produce one another reciprocally.

Why Reflexion within a triangular glass prism is suddenly augmented
when the Light can no longer penetrate.

That bodies which cause greater Refraction also cause stronger
Reflexion.

Demonstration of the Theorem of Mr. Fermat.

CHAPTER IV.
On the Refraction of the Air.

That the emanations of Light in the air are not spherical.

How consequently some objects appear higher than they are.

How the Sun may appear on the Horizon before he has risen.

That the rays of light become curved in the Air of the Atmosphere,
and what effects this produces.

CHAPTER V.
On the Strange Refraction of Iceland Crystal.

That this Crystal grows also in other countries.

Who first-wrote about it.

Description of Iceland Crystal; its substance, shape, and properties.

That it has two different Refractions.

That the ray perpendicular to the surface suffers refraction, and
that some rays inclined to the surface pass without suffering
refraction.

Observation of the refractions in this Crystal.

That there is a Regular and an Irregular Refraction.

The way of measuring the two Refractions of Iceland Crystal.

Remarkable properties of the Irregular Refraction.

Hypothesis to explain the double Refraction.

That Rock Crystal has also a double Refraction.

Hypothesis of emanations of Light, within Iceland Crystal, of
spheroidal form, for the Irregular Refraction.

How a perpendicular ray can suffer Refraction.

How the position and form of the spheroidal emanations in this
Crystal can be defined.

Explanation of the Irregular Refraction by these spheroidal
emanations.

Easy way to find the Irregular Refraction of each incident ray.

Demonstration of the oblique ray which traverses the Crys