Treatise on Light

, etc.

Chap. V. On the Strange Refraction of Iceland Crystal.

That this Crystal also grows in other countries. p. 49.
Who first wrote about it. p. 49.
Description of Iceland Crystal; its matter, shape, and properties. original: "Cristal d’Islande." This is a transparent variety of calcite, famous in the history of science for how it splits light into two beams. p. 49.
That it has two different refractions. p. 51.
That the ray perpendicular to the surface undergoes refraction there, and that rays inclined to the surface pass without refraction. p. 51.
Observation of the refractions of this Crystal. p. 52.
That it has a Regular and an Irregular refraction. p. 54.
The manner of measuring the two refractions of Iceland Crystal. p. 54.
Remarkable properties of Irregular refraction. p. 57.
Hypothesis to explain double refraction. p. 58.
That Rock Crystal also has a double Refraction. p. 59.
Hypothesis of the emanations of light, inside the Iceland Crystal in a spheroidal shape, for Irregular refraction. Huygens proposed that light moved in ellipsoids—like stretched spheres—inside the crystal to explain its odd behavior. p. 60.
How a perpendicular ray can undergo refraction. p. 60.
How the position and the shape of the spheroidal emanations in this Crystal can be defined. p. 62. & 63.
Explanation of Irregular refraction by these spheroidal emanations. p. 63.
Easy way to find the Irregular refraction of each incident ray. p. 66.
Demonstration of the oblique ray, which passes through the Crystal without being broken. In 17th-century physics, "broken" (rompu) was a common term for "refracted." p. 69.
Other irregularities of refraction explained. p. 74.

That an object placed under the Crystal appears double, in two images of different height. p. 77.
Why the apparent heights of one of these images change by changing the position of the eyes above the Crystal. p. 78 and following.
On different Cuts of this Crystal, which produce still other refractions, and confirm this whole Theory. p. 85.
Particular way of polishing its surfaces after it has been cut. p. 88.
Surprising phenomenon concerning rays passing through two separate pieces, the cause of which is not explained. Here Huygens is describing the first observations of the polarization of light, though the concept was not yet fully understood. p. 89.
Likely conjecture on the internal composition of Iceland Crystal, and of what shape its particles are. p. 91.
Proofs to confirm this conjecture. p. 94.
Calculations that were assumed in this Chapter. p. 96.

CHAP. VI. On the Shapes of diaphanous bodies which serve for Refraction and Reflection.

General and easy rule for finding these Shapes. p. 102.
Invention of the Ovals of Mr. DescartesRené Descartes (1596–1650), who formulated early laws of optics. for DioptricsThe study of how light is refracted, especially by lenses.. p. 103.
How he was able to find these Lines. p. 110.
Way to find the surface of a glass for perfect refraction, when the other surface is given. p. 113.
Remark on what happens to rays in the refraction of a spherical surface. p. 118.
Remark on the curved line that forms in the reflection of a concave spherical mirror. p. 123.

Vocabulary used in this section: Iceland Crystal, refraction, double refraction, spheroidal emanations, Dioptrics, Descartes, concave mirror.