of the Electro-magnetic Theory of Light, 345 



given in Fleming's lucid way, points the first finger of the left 

 hand in the direction of magnetic force, the second finger in 

 the direction of electric current, leaving the thumb in the 

 direction of advance of the wave. 



2. Suppose now one endeavors to make a straight-forward 

 step-for-step construction of the electric wave, as is done in the 

 annexed diagram, where full arrows denote magnetic circuits 

 and forces, open arrows the corresponding electric quantities. 



Let Y, Y' .... in the original plane PP of the wave 

 front, denote the direction of increasing electric force, and 

 therefore of electric polarization and of electric displacement 

 currents. Each current is accompanied by a clockwise whirl, 



a, a, . . . , of magnetic force encircling it. These forces neu- 

 tralize each other in the regions between the electric forces, 

 but they have a common direction 7 and y' on either side of 

 the wave front PP. Now let the resulting magnetic forces 7 

 and y' in their turn be treated individually and each provided 

 with a counter clockwise whirl of electric force, polarization 

 and current, as shown by c, e, . . . around 7 and e, e, . . . 

 around y' respectively. The curls so obtained both tend to 

 neutralize the original electric forces Y, Y', . . . , and to 

 replace them by two new planes of electric force or wave 

 fronts Y„ Z"/ . . . and Z 2 , Y/ . . . etc. Clearly electric 

 forces above and below the plane 7, 7' would also tend to neu- 

 tralize each other. 



Each of the new electric forces Y„ Y/ and I z 2 , Y 2 \ ... is 

 however again to be provided with a clockwise magnetic whirl 



b, ... and d, d . . . respectively, which together would 

 tend to neutralize 7 and 7' and evoke the new magnetic forces 

 7, and 7/. Hence by a construction quite recalling Huyghens's 

 principle a new set of electric and magnetic forces Y x and 7,, 

 and Y 2 and 7/ respectively, have been obtained from the origi- 

 nal Y and 7, 7'. The process may be indefinitely repeated, 

 provoking an advance of the wave front in both directions, v 

 and v', from the original wave front PP and at right angle to 

 this plane. In other words, the advance in the figure is to and 

 from the reader. 



But unfortunately both waves, if the forces Zand 7 are 

 correctly drawn, run in the wrong direction ! This may easily 

 be tested by using the left hand as above stated. If the direc- 

 tion of advance v and the electric forces are correct, the mag- 

 netic force is in the wrong direction ; or finally for correct 

 advance and magnetic force, the electric forces turn out nega- 

 tive. True, since we have two waves moving from each other, 

 the discrepancy could be reconciled by coupling the forces of 

 either wave front with the direction of the other, though such 

 counsel is hardly compatible with sobriety. The very predic- 



