Chapter 5
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 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.
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.