62 ]\Ir. S. 1!. lUirbury on Actl(>n 



Tlic onlv analooous vWvvt that 1 have* obsorvtul in iinluro 

 wat; aliDUt twcUc niontlis aoo, when just alter a sunset I ho 

 sH;;litlv hazy but eloudJess eastern sUy in the llunr.es valley 

 near IMaidenlus-ul was of a reniarkahle |)ur[)le hue ; and 1 

 notieed that tre(\s and or;iss relleetino- the liglit from this aspect 

 presented a most unwonted redni^ss of tint. 



VI I J. Lhi Ac' ion. (if a Distance in Dielectrics. 

 Jhi S. 11. lUuinmiY*. 



iV K be the speeitic inductive ca}>aeity of a dielectrie \\\o- 

 dinm, V the potential of any olectritied system, then, 

 aeet)rdin«i; to Trotessor Clerk-Maxwell (Electricity and Mag- 

 netism, vol. i. p. 8t)), the characteristic etpiations of V become, 

 (I) at a point in tlie medium where the volume-density of freo 

 electricity is p, 



("2) at an eUvt riticnl surface wIumv tlie surface-densitv is cr, 



in which ,- is tlu' rale o( incri\ise oi \ per unit of leni'th of 

 dv 



the normal on tlie outsidi^ of tlu^ surface m(\ismvd outwards, 



(/V . 



and -T-j lh(^ incrv\i,s(^ in the same direction inside the surface. 

 dv 



Assumino- tlu^ truth of these ecpiations, it cr.n be sliown that in 

 a heteroocMieous medium the attraction exerted by an electritied 

 partich* at on ant)ther particle at P does not act oenerally 

 in the line (M\ is not o-enerally a function of the distance OP, 

 or of cii-cumstanc(\s existino- at and 1* only, or at points in 

 C)P only. It bcH'omes, then, a matter of irresistible inference 

 tliat a particle at exerts no direct "action at a distance" 

 on the partlcK' at P, but acts u[>on it derivati>c*ly through the 

 medium. 



I'or, let it ho riMpiinnl to lind tlu> force of atiraction at P ex- 

 erted by unit particle at 0. There being no otluM- electritied 

 body in tlu> field except the particle, ecpiation (A) becomes, at 

 all points in sjkicc, 



i:X'\Lrrd7X^^dJ-'d:V^d:)-^ ' ' ^' ^ 

 If, now, K ^^ere corstant, this would lurther rinluce to 



* fonumnuratiHl l>v the Author. 



