78 Intelligence and Miscellaneous Articles. 



have been previously exposed to the most refrangible raye, does 

 not seem here to take place. 



M. Eigollot used his actinometer to investigate the light diffused 

 by the northern region of the sky at different times of the day. He 

 gives the curve obtained on September 17, 1889, on the terrace of 

 the Physical Laboratory of the Faculty of Science at Lyons with a 

 clear sky. This shows a maximum illumination towards half-past 

 twelve ; the curve is virtually symmetrical with respect to this 

 maximum for different times of day. 



The author has investigated if, as M. Egoroff has found for 

 iodized plates, the intensity of the current is inversely proportioual 

 to the square of the distance from the source to the apparatus. 

 This law holds approximately with Drummond's light as source, 

 that is to say with a feeble light, but with sunlight the intensity 

 increases more rapidly than the intensity of the current. 



As the indications of the instrument are proportional to the 

 illumination for faint light, it may be used with advantage in 

 certain cases, such as examining the luminosity of the sky in 

 diffused light. — Journal de Physique, November 1892 ; from 

 Annates de Cliimie et Physique, vol. xxii. p. 567, 1891. 



0^ THE ATTRACTION OF TWO PLATES SEPARATED BY A 

 DIELECTRIC. BY M. JUL1EN LEFEYRE. 

 I have measured the attraction of two electrified plates sepa- 

 rated by a dielectric not in close contact with them, and have 

 verified that it is represented by the following formula : — 



F/e-pV V 



where F' is the attraction of the two plates at the distance e-\-e in 

 air ; F the attraction at the same distance when a plane insulating 

 plate of thickness e has been placed between the plates ; e' is then 

 the sum of the thicknesses of air on either side of this plate ; k is 

 the dielectric constant of the plate. 



I used two horizontal plates, and a sensitive balance to the 

 beam of which was hung at one end a scale-pan ; the movable 

 plate of the electrical apparatus, which is 12 centim. in diameter, 

 is suspended at the other end by an insulating stem. This plate 

 is surrounded by a guard-ring provided with a kind of cover, which 

 is only perforated by a hole to allow the rod to pass. 



The fixed plate, which is 19 centim. in diameter, is placed below 

 the former and rests on an insulating support with levelling- 

 screws, so that the distance e-\-e can be varied. Three insulating 

 rods pass through this plate and support the dielectric. On either 

 side of this are layers of air as thin as possible ; it must, however, 

 be possible for the movable plate to make small oscillations. 



The source of electricity is a Euhmkorff's coil worked by six 

 Bunsen's elements. One of the poles communicates with the 

 fixed plate, and with the inner coating of a jar the capacity of 



