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 Crystal without
being refracted.

Other irregularities of Refraction explained.

That an object placed beneath the Crystal appears double, in two
images of different heights.

Why the apparent heights of one of the images change on changing the
position of the eyes above the Crystal.

Of the different sections of this Crystal which produce yet other
refractions, and confirm all this Theory.