Intelligence and Miscellaneous Articles, 319^ 



rotation is augmented ; so that, in the conditions under which they 

 operated, the alteration of luminous intensity did not sensibly modify 

 the results of their observations. Still it would be desirable that 

 the authors should be able to explain in what manner the luminous 

 intensity intervenes when comparisons are made between sparks of 

 unequal brightness, especially when the discharges explode between 

 electrodes of various metals, placed at different distances, in gases 

 at divers pressures, and that it should be possible for them, in cer- 

 tain cases, to operate with the same sum of light illuminating the 

 lines of their apparatus. 



Since the coincidences of the lines of the fixed and moveable disks 

 depend on the duration of the spark up to a certain limit of lumi- 

 nous intensity, it must be remarked that their number might be 

 increased in consequence of the phosphorescence of the moveable 

 disk ; but mica being one of the solids in which the phenomenon of 

 persistence of this luminous action is the least marked, it follows 

 that no sensible perturbation can arise from the interposition of a 

 moving disk of that substance between a luminous focus and the eye 

 of the observer. 



The authors of the memoir have not been able, with the apparatus 

 constructed as it is, to render appreciable the duration of a spark 

 proceeding from an ordinary machine ; but they have ascertained 

 that the duration of the discharges from condensers varies with the 

 surface of these, with their disposition, and in proportion to the re- 

 sistance of the circuit traversed by the electricity ; it changes like- 

 wise with the exploding-distance, the nature of the knobs of the 

 exciter, and the humidity of the air. In general the duration 

 increases with the surface of the condenser and with the distance of 

 the knobs, and diminishes with the lengthening of the circuit. In 

 these researches, they have given as limits of observed durations 

 4 millionths of a second and 86 millionths of a second, with a pos- 

 sible error of 1 millionth of a second. 



They have been able to represent by empiric formulse the results 

 obtained in divers series of observations, and have arrived at this 

 consequence — that there is a limit towards which the duration of 

 the spark tends when one augments indefinitely the surface of the 

 condenser and the exploding-distance and diminishes the resistance 

 of the conducting circuit. 



Briefly, MM. Lucas and Cazin have devised an ingenious method 

 of experimenting, which they have studied with care, and which has 

 already led them to some very interesting results in the experiments 

 made with condensers ; but it would be important to render this 

 method equally applicable to the investigation of the duration of 

 sparks produced by ordinary machines without the intervention of 

 batteries. The Commission, therefore, invite MM. Lucas and Cazin 

 to continue their researches, and have the honour to propose to you 

 to order the insertion of their memoir in the Recueil des Savants 

 etrangers. 



The conclusions of this Report were adopted by the Academy. — 

 Comptes Rendus de VAcad, des Sciences, July 8, 1872, pp. 66-69. 



