Among the optical applications we may cite Mr. Leon Laurent’s apparatus for controlling plane, parallel, perpendicular, and oblique surfaces, and magic mirrors obtained with an ordinary light; Mr. S.P.  Thompson’s apparatus for demonstrating the propagation of electro-magnetic waves in ether (according to Maxwell’s theory), as well as some new polarizing prisms; and a mode of lighting the microscope (presented by Mr. Yvon), that was quite analogous to the one employed more than a year ago by Dr. Van Heurck, director of the Botanical Garden of Anvers.

Acoustics were represented by an electro-magnetic brake siren of Mr. Bourbouze; Konig’s apparatus for the synthesis of sounds; and Mr. S.P.  Thompson’s cymatograph—­a pendulum apparatus for demonstrating the phenomena of beats.

It was electricity again that occupied the largest space in the programme of the session.

Apparatus for teaching are assuming greater and greater importance every day, and the exhibit of Mr. Ducretet included a large number of the most interesting of these.  The house of Breguet exhibited on a reduced scale the magnificent experiments of Gaston Plante, wherein 320 leaden wire secondary elements charged for quantity with 3 Daniell elements, and afterward coupled for tension, served to charge a rheostatic machine formed of 50 condensers coupled for quantity.  These latter, coupled anew for tension, furnished upon being discharged a spark due to a difference of potential of about 32,000 volts that presented all the characters of the spark produced by induction coils on the machines so improperly called “static.”  Finally, we may cite the apparatus arranged by Mr. S.P.  Thompson for studying the development of currents in magneto-electric machines.  The inventor studies the influence of the forms of the inductors and armatures of machines by means of an arrangement that allows him to change the rings or armatures at will and to take out the induced bobbins in order to sound every part of the magnetic field.  Upon giving the armature an angular motion limited by two stops, there develops a certain quantity of electricity that may be measured by causing it to traverse an appropriate ballistic galvanometer.  Messrs. Deprez and D’Arsonval’s galvanometer answers very well for this purpose, and its aperiodicity, which causes it quickly to return to zero as soon as the induced current ceases, permits of a large number of readings being taken within a very short space of time.

Measuring apparatus were represented by a new and very elegant arrangement of Sir William Thomson’s reflecting galvanometers, due to Mr. J. Carpentier.  The mounting adopted by Mr. Carpentier permits of an easy removal of the bobbins and of an instantaneous substitution therefor.  The galvanometric part, composed of the needles and mirror, therefore remains entirely free, thus allowing of its being verified, and making it convenient to attach the silken fiber.  Mr. Carpentier has, moreover, adopted for all the minor apparatus a transparent celluloid scale which simplifies them, facilitates observations, and renders the use of reflection almost industrial.