Christiaan Huygens had visited London in 1689,…

Christiaan Huygens had visited London in 1689, met Isaac Newton, and lectured on his own theory of gravitation before the Royal Society.

Although he did not engage in public controversy with Newton directly, it is evident from Huygens' correspondence, especially that with Gottfried Leibniz, that in spite of his generous admiration for the mathematical ingenuity of the Principia, he regarded a theory of gravity that was devoid of any mechanical explanation as fundamentally unacceptable.

His own theory, published in 1690 in his Discours de la cause de la pesanteur (“Discourse on the Cause of Gravity”), though dating at least to 1669, included a mechanical explanation of gravity based on Cartesian vortices.

Huygens' Traité de la Lumière (Treatise on Light), by 1678 already largely completed, was published in 1690 also.

In it he again showed his need for ultimate mechanical explanations in his discussion of the nature of light, but his beautiful explanations of reflection and refraction—far superior to those of Newton—are entirely independent of mechanical explanations, being based solely on the so-called Huygens' principle of secondary wave fronts.

Huygens' principle, a statement that all points of a wave front of light in a vacuum or transparent medium may be regarded as new sources of wavelets that expand in every direction at a rate depending on their velocities, is a powerful method for studying various optical phenomena.