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.
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