doctrine as subsequently developed by Thomas Young and Augustin
Fresnel, and belonged rather to geometrical than to physical Optics.
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.