8 MM. E. du Bois-lleymond and W. Beetz on ike 



lead in caustic potash upon a plate of German silver (? " Mail- 

 lechort"); the free extremity of an extremely fine platinum- 

 wire drawn out by Wollaston's method, and fused in a glass 

 tube, is dipped into the liquid opposite the middle of the plate, 

 the wire is then connected with the negative and the plate with 

 the positive pole of a battery composed of eight or ten pairs. 

 A delicate film of peroxide of lead is deposited upon the 

 plate, and as the current is stronger in the direct line from 

 the point towards the plate than in the lateral directions, this 

 film diminishes in thickness from the centre to the circumfer- 

 ence, and hence exhibits Newton's rings most splendidly. M. 

 Becquerel remarks, that as the index of refraction of the 

 peroxide of lead lies between that of the air and that of the 

 metal, the rings in the present instance should be compared 

 to those appearing in transmitted light rather than to those 

 obtained by reflexion. In fact it appears that the arrange- 

 ment of the series of colours in Nobili's rings from without 

 inwards, very accurately corresponds to that from within out- 

 wards in Newton's rings by transmitted light. The next point 

 is to prove that the relative thicknesses of the film, which may 

 be ascertained from the laws of diffusion of the electric current, 

 in conjunction with the law of electrolysis, agree with those 

 which, as regards the light and dark parts of the rings, are 

 readily deduced from optical considerations. 



Speaking of the law according to which the thickness of 

 the rings varies, M. Becquerel says, " It is easy to Jind the 

 value of the curve formed by the upper surface of the film of 

 oxide, in a plane perpendicular to the metallic plate and passing 

 through its centre. Let BX . . . (see PI. I. fig. 1) be the metallic 

 plate, B its centre, A the extremity of the negative point of the 

 pole, and let us suppose that the electricity escapes by that point 

 only. Let C be any point; lichen the current passes, the thickness 

 of the layer of oxide deposited at the point C will depend entirely 

 upon the intensity of the current and the distance A C. Ad- 

 mitting that this intensity is in inverse proportion to the distance 

 AC, we should have 



_ A 



as the equation of the curve which limits the upper part of the 

 film of oxide, m being the distance A B, and A a coefficient de- 

 pending upon the conducting power of the solution and the power 

 of the battery. If care be taken to place the point A very near 

 B, within a half millimetre, m % may be omitted before x\ and 

 the equation becomes 



A 



