IT may perhaps interest your readers to know that the 200th anniversary of the publication of Newton's “Principia” was solemnly celebrated on December 20 (old style) by a united meeting of two learned Societies of Moscow—the Imperial Society of Friends of Natural Knowledge, and the Mathematical Society. Prof. Mendeleeff, of St. Petersburg, was Honorary President. Prof. Stoletow (President of the Physical Section in the first-named Society) presented a sketch of Newton's life, and spoke on his optical discoveries; Prof. Zinger (President of the Mathematical Society) treated Newton's mathematical work; Prof. Joukowski pointed out his merits as founder of rational dynamics; and Prof. Ceraski exhibited the creation of celestial mechanics by Newton. The large hall of the Polytechnic Museum, where the meeting took place, was attended by the elite of the city. The lectures were illustrated by some optical experiments with electric light and some lantern-slides relative to Newton's biography.