AN OBSTACLE ON A TRAIN OF ELECTRIC WAVES. 
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the conducting surface, and, if the point P is internal to the tangent cone from O to 
the conductino- surface and on the side of the conducting surface remote from the 
point O, the point for which i\ + r is stationary is the point at which the straight line 
OP cuts the conducting surface nearest to the point 0. The values of the principal 
parts of a, fB, y are therefore a,,, /So, Jq, where 
“o = {(^lo + WT^o + nCo) {y-Vo) + n (770Z-C0?/)} 
^0= - W^o + mrjo + nCo) {x-^o) + n {^oX-Cox)} 
yo = - 2 ^ ^ ivoX-^oy) 
where {^ 0 , 7]o, { 0 ) ai’e the co-ordinates of the point Q for which }\ + 7' is stationary, 
I, m, n now denote the direction cosines of the outward drawn normal to the surface 
at the point Q, I is the principal value of the integral 
and 
R,’> = f.n 
To calculate the value of I, it is convenient to choose for axes of reference the 
normal to the surface at the point Q (^ 0 , yjQ, 4) as the axis of the tangent to the 
surface in the plane of incidence as the axis of and the perpendicular tangent as 
the axis of 17 . Let the equation of the surface referred to these axes be 
2 ^ = k^^+'2R^'q + Bri^+ 
let (iCi, 0 , Zi) be the co-ordinates of the point O, and let {x^, y^, be the co-ordinates 
of the point P referred to these axes, then 
and further 
n ^ + (Af+H,+ ...), 
n|5 = >) + (£-2.) (Hf+B,+ 
