The 'Coamotheoros ' was passing through the press when Huyghens died. It was printed at the Hague in 1698, and was twice printed in English, first in 1693, and next at Glasgow in 1757; besides several translations into continental languages. It defends tho Copernicau system, and enters into a large number of speculations on the physical constitution and probable inhabitants of the planets.
IV'. Optical Works.
These are—the 'Traitd de la. Lumiere,' Leyden, 1690, Latinised in the ' Opera Reliqua ; the Dioptrics, and the ' Commentarii de Vitris Figurandis,' both first given iu the posthumous works. The first treatise was reprinted by Baron 31aseres in his ‘Scriptores Optici,' London, 1823. It was written in 1678, and must now be considered as the 'Principle' of optics. Huyghens took up the theory of undu lations in opposition to that of emanation, which was adopted by Newton. By this theory he gave a sufficient explanation of the pheno mena of reflexioo and refraction, and also of that of double refraction, in which Newton could not succeed ; that is, he gave an explanation of all the prominent phenomena of optics. The undulatory theory is now almost universally received, and Huyghens must be considered as the founder of it ; for though Hooke had previously advanced the notion, yet he made no application of it to the explanation of phenomena.
It remains to mention the treatise 'Da Ratiocin i is in Ludo Aleas,' which was printed at the end of Schooten's Exereitationes Mathematicm; Leyden, 1657. It is the earliest regular treatise on questions of chances, and first points out the manner in which the expectation of a player is determined. Some minor writings we leave unnoticed.
As a philosopher, Huyghens is distinguished by correctness, pene tration, and a freshness of intellect which never left him. Before he was in possession of the formal differential calculus he was able to supply its place. His power of acquisition lasted to the end of his life. He was near sixty when he read the 'Principle,' and past that age when he began to study the Calculus of Leibnitz. At that time of life persons seldom change old opinions, but Huyghens admitted the theory of Newton instantaneously; and he was probably the first cootioental philosopher who published his adhesion to the theory of gravitation, not generally, but after miuute examination.