Theory of Nobili's Coloured Rings. 



23 



(In) is infinitely small compared with the radii, we do not 



have n.r*= const., but w(r 2 + ?» 2 ) 5 = const. In the above 

 experiments the distance m is entirely neglected. I therefore 

 made some experiments in which the point of the cathode was 

 first placed in immediate contact with the anode, and then 

 by a support, the clamp which supported the anode was 

 moved, and the elevation measured. In these experiments 

 the rings augmented in breadth as m increased, and conse- 

 quently were less numerous. In the case of the value w=qo 

 the plates obtained must naturally be of one perfectly uniform 

 colour, and in fact, by removing the cathode quite away from 

 them, we can cover small plates with a tolerably equal coating. 

 These colours are still better obtained when the point is 

 placed at a considerable distance above the plate ; it is not 

 advisable to replace it by a plate, because the precipitate is 

 then formed very violently and with the colour of the peroxide 

 of lead itself, whilst with a less intensity of the current used 

 we can regulate its formation. 



With regard to m I obtained the following results with two 

 plates : — 



Table XIV. 



Table XV. 



The approximation in the case of the second dark ring is 

 thus perfect, and even in the first light one. tolerably percep- 

 tible. Further distances of the point from the plate than those 

 detailed, were not applicable, because the error which arises 

 from its finiteness must then constantly become visible. 



