If Huygens had no conception of transverse vibrations,
of the principle of interference, or of the existence
of the ordered sequence of waves in trains, he nevertheless
attained to a remarkably clear understanding of the
principles of wave-propagation; and his exposition
of the subject marks an epoch in the treatment of Optical
problems. It has been needful in preparing this
translation to exercise care lest one should import
into the author’s text ideas of subsequent date,
by using words that have come to imply modern conceptions.
Hence the adoption 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
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.
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