484 DR G. PLARR ON THE DETERMINATION OF 



By the juxtaposition of xpi and xjjfy- 1 ?.), 



»^-'0=y^-|) + ,(_5; + £.). 

 we remark that we have 



+J [^-i)+^(_5 + i)] 



This accounts for the fact that 



becomes identically zero, because ^ and £ are perpendicular to one another by 

 the definition of £ 1# Hence 



It follows that for any line co directed in the plane of i and of <f>- l i, namely 

 in the plane normal to the ellipsoid at the extremity of Pl and containing the 

 vector p v the results of 



will solely give the equation 



W 

 2— 1 = 0. 



u 



Let us now form the result 



As <f)i differs generally in direction from i and from <£ - H we may expect an 

 equation distinct from the preceding one. 

 We have 



k 

 S</>i.k<f>- l k. 



