1871.] Light. 413 



LIGHT. 



At the recent soiree of the Royal Society, Mr. J. Browning exhibited a spec- 

 troscope, in which the rays of light pass through two batteries of prisms, and 

 were then sent back through both trains of prisms, by means of a reflecting 

 prism placed behind the last prisms of the train, to the eye of the observer. In 

 this manner a dispersive power equal to nineteen prisms of flint-glass is 

 obtained. Both trains of prisms, as well as the intermediate prism, are under 

 the control of an automatic movement, which ensures that every ray shall pass 

 at the minimum angle of deviation, for the particular ray under examination. 

 Both the collimator and telescope are fixed; the prisms only are movable. 

 Mr. Proctor suggested this extension of Mr. Browning's automatic spec- 

 troscope, and contrived the form of prisms which would enable the automatic 

 action to be carried on, but the mechanical difficulties, in communicating 

 the automatic motion to the second battery were very considerable. These 

 difficulties Mr. Browning has ingeniously surmounted. The D lines in the 

 spectrum of sodium are seen in this instrument, separated by an apparent 

 interval of more than £ of an inch, and under favourable conditions of the 

 atmosphere, ten or twelve lines are visible between them. We understand 

 that Mr. Spottiswoode, the Treasurer of the Royal Society, has become 

 the possessor of this instrument. 



The subject of signalling communications between fortresses and distant armies 

 has occupied the attention of our French neighbours for a considerable time. 

 M. Le Verrier has recently drawn attention to a lengthy paper containing the 

 record of a series of experiments made for the Chief Committee of Defence of 

 the Valley of the Rhone, Lyon, for the purpose of communicating, by 

 luminous signals, at distances of 30 to 100 kilometres. It appears that, 

 under the guidance of the well-known savant just named, excellent results 

 have been obtained, and so simple in execution that ordinary workmen 

 can satisfactorily manage the signals. The considerable expenses of these 

 researches have been defrayed by a private individual, M. Maistre, of Villeneu- 

 vette, near Clermont, Herault. 



M. Delaurier, of the . French Academy of Science, has taken up the 

 proposal of M. Felix Lucas, a Government engineer, to obtain a very powerful 

 electric light for coast service, at a comparatively small cost, by making 

 the light intermittent, with intervals of two seconds ; by this means, it is said, 

 one 10,000th part of the electricity expended for a constant light will 

 suffice. M. Lucas proposed to use static, but M. Delaurier prefers dynamic, 

 and especially induced electricity, and suggests the following arrangement : — 

 A battery which is only in action when the circuit is open, the couples united 

 together in series, and the electricity passed through coils of solid iron wire ; 

 a very long copper wire, covered with silk, is wound around the iron wire-coil, 

 the copper being sufficiently thick not to be heated by the passage of the cur- 

 rent. Each pole is provided with a charcoal point ; when these are in contact 

 the current passes, and when they are separated the sparks are excessively 

 brilliant, because both the direct and the extra current are brought into 

 action, resulting from the induction of the current on itself and that by 

 the coil of iron wire. The charcoal points are favourable to a spark of 

 short duration. A very simple arrangement of clockwork causes the points to 

 come into contact every two seconds, and then separates them sharply, so as 

 to break the current instantaneously, It is not possible to obtain by these 

 means a flash of such short duration as with static electricity, because the 

 current passes for an instant through the air when the charcoal points are 

 separated. It is believed that this system will be found useful, not only 

 for piercing fogs at sea, but also for railway signals. 



C. Schultz-Sellack has communicated a series of experiments with the 

 object of proving that, while the chloride, bromide, and iodide of silver are 

 acted upon by light, and altered in a well-known manner, this action is due to 

 a phenomenon of dissociation, called forth by the light ; but when these salts 

 are put in sealed tubes, and care taken to have an excess of the haloids 



