562 Progress in Science. [Odlober,; 



■ * 



mirror, a b, placed at an angle of 45 , and after reflection from it falls upon the 

 third lens placed horizontally at c. This concentrates it upon the objective at 

 e, from which it passes to the mirror f g, and is so reflected to the screen. 

 This mirror, moreover, is not silvered on the exterior surface, but in the usual 

 way, though with pure silver on the back. Yet no want of definition is to be 

 perceived in the image, owing no doubt to the fact that the faint reflection from 

 the first surface is inappreciable in comparison with that from the metallic 

 silver. In several articles published in the " Chemical News" and elsewhere, 

 a square prism has been described as being used for the same purpose. But 

 this arrangement leaves a third of the field dark, because about one-third of 

 the cone of rays entering the prism is at an angle too great for total reflection. 

 To exhibit magnetic spectra, a plate of glass is placed upon the third lens of 

 the condenser, iron filings scattered evenly upon it, a small steel magnet 

 placed beneath, and the glass lightly tapped with a pencil point. The various 

 phenomena of wave motion, interferences, reflexions, &c, are demonstrated 

 by Professor Morton, with an apparatus contrived by Messrs. Hawkins and 

 Wale, the instrument makers, who satisfactorily carried out his instructions 

 with regard to the making of the lantern itself. It consists of a metallic box 

 with a sheet-rubber cover, provided with a long metal tube ; this is so placed 

 that the tube is about one quarter of an inch above the point in the tank 

 which it is desired to make the centre of the wave motion. On tapping the 

 rubber diaphragm a momentary puff of air is driven from the tube, producing 

 exactly the disturbance needed. By placing an elliptical ring inside the tank, 

 the reflexions, interferences, &c, are correspondingly modified. 



By letting fall drops of ether, alcohol, carbolic acid, oil of cinnamon, coriander, 

 cloves, &c, on the surface of water contained in the tank, cohesion figures are 

 produced, which may be admirably exhibited in a lantern of this description, 

 and so likewise the electric decomposition of metallic solutions. 



Another experiment of peculiar beauty consists in attaching a ring of thin 

 rubber, 5 inches in diameter, to a corner segment of a glass Chladni plate 12" 

 square, and filling the ring about \ inch deep with water. On vibrating the 

 plate with a bow (the corner segment being of course in the lantern field), the 

 area is filled with the most beautiful crispations, changing with the tone as the 

 harmonics are sounded. This experiment, as we witnessed it in the Academy 

 of Music, was of most unusual beauty. 



Stevens Institute of Technology, U.S., 

 July 15, 1871. 



SCIENTIFIC INVENTIONS AT THE INTERNATIONAL 

 EXHIBITION OF 1871. 



SCIENCE is well represented at the present Exhibition, although the inven- 

 tions and models to be viewed are not numerous. Each, however, either 

 embodies the application of some highly scientific and known principle, or is 

 the result of much thought and labour in a new field. There is an improve- 

 ment in the arrangement this year much to be commended — the actual 

 working of every model — rendering the scientific department not only more 

 attractive to the uninitiated, but greatly assisting the comprehension of the 

 invention. The general aspect of the Exhibition and Royal Albert Hall is well 

 known from descriptions in the daily press ; and we can at once proceed to 

 chronicle that which is new in a technical sense. Messrs. Siemens, as usual, 

 have contributed largely. The Pyrometer, by C. W. Siemens, F.R.S., has 

 already been noticed in the pages of this journal. Dr. Werner Siemens 

 exhibits two of his electrical inventions — the Electrical Distance Meter, an 

 apparatus for obtaining a correct estimate of the position of distant objects, 

 applicable more especially to the marking of the course of a hostile vessel 

 approaching a battery of torpedoes, and to similar purposes. The other in- 

 vention is the Dynamo-Electric Mine Exploder, the principle of which was 

 described in a paper to the Royal Society in February, 1867. Mr. C. W. Sie- 

 mens also contributes a model of furnaces for the production of cast-steel from 



