of as literal a rendering as possible.
A few of the author's terms need explanation. He uses the word
"refraction," for example, both for the phenomenon or process usually
so denoted, and for the result of that process: thus the refracted ray
he habitually terms "the refraction" of the incident ray. When a
wave-front, or, as he terms it, a "wave," has passed from some initial
position to a subsequent one, he terms the wave-front in its
subsequent position "the continuation" of the wave. He also speaks of
the envelope of a set of elementary waves, formed by coalescence of
those elementary wave-fronts, as "the termination" of the wave; and
the elementary wave-fronts he terms "particular" waves. Owing to the
circumstance that the French word _rayon_ possesses the double
signification of ray of light and radius of a circle, he avoids its
use in the latter sense and speaks always of the semi-diameter, not of
the radius. His speculations as to the ether, his suggestive views of
the structure of crystalline bodies, and his explanation of opacity,
slight as they are, will possibly surprise the reader by their seeming
modernness. And none can read his investigation of the phenomena found
in Iceland spar without marvelling at his insight and sagacity.

S.P.T.

June, 1912.

TABLE OF MATTERS

Contained in this Treatise

CHAPTER I.
On Rays Propagated in Straight Lines.

That Light is produced by a certain movement.

That no substance passes from the luminous object to the eyes.

That Light spreads spherically, almost as Sound does.

Whether Light takes time to spread.

Experience seeming to prove that it passes instantaneously.

Experience proving that it takes time.

How much its speed is greater than that of Sound.

In what the emission of Light differs from that of Sound.

That it is not the same medium which serves for Light and Sound.

How Sound is propagated.

How Light is propagated.

Detailed Remarks on the propagation of Light.

Why Rays are propagated only in straight lines.

How Light coming in different directions can cross itself.

CHAPTER II.
On Reflexion.

Demonstration of equality of angles of incidence and reflexion.

Why the incident and reflected rays are in the same plane
perpendicular to the reflecting surface.

That it is not needful for the reflecting surface to be perfectly
flat to attain equality of the angles of incidence and refle