2T 



treatise on Magneto- and Electro-Optics ^). On page 35 of this book 

 a formula (p = vd^ occurs which (allowing for change of notation) 

 will be seen to agree with our above equation (2) in Art. 1; but 

 its deduction, on pp. 34 and 35, rests upon the explicite assumption 

 that a quantity vd^ (depending on the difference x^ — x^, in our 

 notation) is very small in comparison to unity; and on page 35 

 the result (pz=vdi is adduced with express reservation to the case 

 when the absorption is feeble ^). Considerable uncertainty thus arises 

 as to the accuracy and generality of the proposition in question; 

 and as this matter touches so vitally one of the main foundations 

 of the theory, I have thought that it might be worth while to show 

 how, by merely analytical transformation, and without the help of 

 any subsidiary assumption whatever, the result (p = vd^ can be 

 deduced from Professor Voigt's own system of general equations. 



To avoid confusion I shall follow Professor Voigt's notation;, 

 its use will be restricted, however, to the present Article only. 



We start from the perfectly general formula (35) on page 34 

 of « Magneto- und Elektrooptik* 



V, 



3«V p — «V 



C/i «(e«vS_j_g-.-vS)- 

 We have to take here 



(1) 



d = di — i ^2 . (2) 



(3) 



(1 — y^) sin v6^ cos vd^ — iy 

 F— (1 — 72) sin2 vd^ 



(4) 



(5) 



Going back to Professor Voigt's equations (24), on page 31, we 

 gee that 



1) Magneto- und Elektrooptik, von Dr. Woldemar Voigt, Leipzig 1908. 

 *) «Bei, wie vorausgesetzt, kleiner Absorption*. 



