ion
of its famous property which is the main foundation of the
construction of our eyes and of those great inventions which extend so
vastly the use of them.

I hope also that there will be some who by following these beginnings
will penetrate much further into this question than I have been able
to do, since the subject must be far from being exhausted. This
appears from the passages which I have indicated where I leave certain
difficulties without having resolved them, and still more from matters
which I have not touched at all, such as Luminous Bodies of several
sorts, and all that concerns Colours; in which no one until now can
boast of having succeeded. Finally, there remains much more to be
investigated touching the nature of Light which I do not pretend to
have disclosed, and I shall owe much in return to him who shall be
able to supplement that which is here lacking to me in knowledge. The
Hague. The 8 January 1690.

NOTE BY THE TRANSLATOR

Considering the great influence which this Treatise has exercised in
the development of the Science of Optics, it seems strange that two
centuries should have passed before an English edition of the work
appeared. Perhaps the circumstance is due to the mistaken zeal with
which formerly everything that conflicted with the cherished ideas of
Newton was denounced by his followers. The Treatise on Light of
Huygens has, however, withstood the test of time: and even now the
exquisite skill with which he applied his conception of the
propagation of waves of light to unravel the intricacies of the
phenomena of the double refraction of crystals, and of the refraction
of the atmosphere, will excite the admiration of the student of
Optics. It is true that his wave theory was far from the complete
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