SCIENTIFIC.

Otago Witness, Issue 1917, 13 November 1890, Page 41

SCIENTIFIC.

Seeing By Electricity.

It is said that modern electricians may sticceed in constructing a device that will do for the sense of sight what the telephone does' for the sense of hearing ; brib the prospects of such an achievement are not particularly bright. The possibility has been demonstrated oxrerimentally, bnt the practical difficulties in the way c f carrying out the idea are very great.

It is well known that certain salts of copper and silver, for example, generate electric currents when exposed to the light, and conversely electric currents will prodece in these salts chknges corresponding to those, madfl by light. If a strongly illuminated object is placed in front of a transmitter covered with such a salt electrical currents will be generated which will vary in intensity according to the intensity of the light falling on differrents parts of the surface. If now these various currents were carried to a receiver similar to the transmitter, they would produce on its surface changes like those wrought by light on the surface of the transmitter.

Another apparatus has been proposed, based upon the property peculiar to selenium that its electrical resistance changes with the intensity of light. For a transmitter the selenium cells are arranged in squares, like a checker board. The greater the number the more efficient would the apparatus be. Each of these is connected with an electro magnet in the receiver. The latter is composed of a mirror constructed of thin,strips of steel, to each of which several of the electro-magnets are attached. A strongly illuminated body beibg placed in front of the selenium transmitter, the electrical resistance of the various cells varies according to the intensity of the light, as in the case of the salts of copper and silver. The various currents affect the electro magnets in different degrees, the result being that the strips of steel are bent irregularly, turning the mirror into an aggregation of surfaces, very slightly warped, which will throw an imago on a ecreen.

The main difficulty in the way of the realisation of such an idea as seeing by electricity lies in the necessary complexity of an apparatus which shall enable innumerable points in the transmitters and receivers to be actuated simultaneously in perfect independence and also in perfect correspondence ; but it is quite possible that further experiment will bring into actual use what might be something more than a mere plaything. In defensive warfare a telescope of indefinite length would certainly be of value. With buried wires and concealed visors a general might watch his adversary's movements at times when this would mean victory instead of defeat. — Electrical World.

A Tidal Supply of Electricity.— A French engineer, M. Decoaur, proposes to supply electric power to Paris. He would generate the required electricity by utilising the tides. For this purpose he intends to construct, near Havre, two large basins joined to each other, into one of which the sea at flood tide flows over a dam, while during ebb it flows out of the other into the sea again. At the inlet and outlet will ba erected a number of powerful turbines for transmitting the energy of the water. The mechanical energy produced for transmission to Paris is estimated by M. Decoeur at 42,000 borse-power. Perhaps the calculations have not been correctly made, as it costs something to build such work, and the result cannot be much. — English Mechanic. Scientific Excubston to Mont Bland. —Intelligence '«ms been received in Paris that M. Janssens, the astronomer and member of the French Institute, who left a short time ago on a scientific excursion to Mont Blanc, reached the Grands Mulcts on the 17th June. On the following day a party of 15 guides and porters took charge of the astronomer, who is in delicate health, and took him in a sledge specially built for the occasion up to the Chalets dcs Bossons, at an altitude of 14,600 ft, by w^y of the lesser and greater plateaus and the Bosses tin Drotnadaire. A small scientific laboratory, provided with the necessary meteorological instruments, has been set up in the Bo«sons hut, where M. Janssens will carry on a series of observations in spectral analysis.

CeJTENTING £8 A SUBSTITUTE FOR Wklijing. - By a new method of cementing iron the parts cemented are so effectually joined as to resist tbe blows even of a sledge

hammer. The cement is composed of equal parts of sulphur and white lead, with a proportion of about one-sixth of borax When the composition is to be applied it is wet with strong sulphuric acid, and a thin layer of it is placed between the two pieces of iron, which are at once pressed together. In five days it will be perfectly dry, all traces of the cement having vanished and the work having every appearance of welding.