American Journal of Science, October.—Recent progress in optics, by W. Le Conte Stevens. This paper was read before the American Association, and gives an admirable summary of Michelson's work with the interferential comparer, of Wiener's experiments with stationary light waves, of recent researches on luminescence, and other subjects.—The quantitative determination of perchlorates, by D. A. Kreider. The method is essentially the collection of the oxygen of the perchlorate; its subsequent passage into an atmosphere of nitric oxide over a strong solution of hydriodic acid, and the titration of the iodine thus liberated with decinormal arsenic in alkaline solution. The apparatus employed consisted of a piece of combustion tubing, 10 or 12 cm. in length, drawn out at one end and connected with a receiver filled with caustic potash. A platinum boat carried the perchlorate, which was covered with a mixture of sodium and potassium carbonates. The tube was then filled with carbonic acid, and then the oxygen was evolved by fusion. The oxygen was swept by a current of CO2 into a receiver consisting of two levelling bottles. For the action of the oxygen on hydriodic acid through the medium of nitric oxide, a bulb pipette was used with stopcocks at both ends. This was filled with a known amount of hydriodic acid, and the air expelled by CO2, after which it was exhausted and a small quantity of nitric oxide was admitted. Then the oxygen was allowed to enter slowly under the diminished pressure, while the bulb was constantly shaken. It was then removed for titration.—Demonstration of caustics, by R. W. Wood. A strip of thin polished steel is bent into an arc formed by pins stuck on a board. A piece of cardboard is placed across the opening of the arc, and slits are cut in it about half an inch apart. A piece of photographic sensitive paper is stuck on a board inside the arc. On directing the apparatus towards the sun, parallel rays are traced from the slits to the mirror, and also their reflections, and the latter form the caustic surface appropriate to the curvature of the mirror.—The law of electromagnetic flux, by M. I. Pupin. The author endeavours to show the exact position which this law occupies in Maxwell's electromagnetic theory; to point out its limitations; to show that Maxwell's electromagnetic theory of light demands a more general form of this law; and to present a general form of this law of which the forms given up to the present are special cases.