But rays of light pass through it in another fashion and produce those marvellous refractions the causes of which I am now going to try to explain; reserving for the end of this Treatise the statement of my conjectures touching the formation and extraordinary configuration of this Crystal.
6.

In all other transparent bodies that we know there is but one sole and simple refraction; but in this substance there are two different ones.

The effect is that objects seen through it, especially such as are placed right against it, appear double; and that a ray of sunlight, falling on one of its surfaces, parts itself into two rays and traverses the Crystal thus.
7.

It is again a general law in all other transparent bodies that the ray which falls perpendicularly on their surface passes straight on without suffering refraction, and that an oblique ray is always refracted.

But in this Crystal the perpendicular ray suffers refraction, and there are oblique rays which pass through it quite straight.
[Illustration] 8.