84 



Mr. F. T. Trouton on the Influence of 



The following table served for plotting the curve. In the 

 first column are the dimensions of the reflector in the direc- 

 tion of the electric component. Six determinations were 

 made of the position of the node with each sized reflector. 

 The mean of these is given in the last column. The reading 

 taken was in each case the nearest quarter centimetre, the 

 nature of the observations making further exactitude un- 

 necessary. The effect from a 12 cm. reflector was too weak 

 to admit of an observation. 



Table I. 



16 



20 



24 



28 



32 



36 



48 ..... 



60 



75 



Large 1 

 sheet J 



24f 



22 



21 



21| 



20* 



20* 



19 



17| 



17 



17| 



23| 



22* 



22* 



20-1- 



20* 



20 



19* 



18 



17 



16* 



22* 

 23* 

 22| 

 21| 

 21* 

 20| 

 19* 



m 



17 

 174 



24f 

 22* 

 21* 



m 



21* 



21 

 18| 



18| 

 16| 



17 



w<± 2 



24! 



24-2 



22! 



22* 



22-5 



21f 



21! 



21-8 



20! 



21* 



21-2 



20* 



20* 



20-7 



20* 



20* 



20-5 



18* 



18* 



18-9 



18* 



17* 



18 



17! 



17* 



17-2 



16! 



IT* 



17 



The effect produced in the reflected wave by shortening 

 the reflector in the direction of the electric component can 

 be considered equivalent to an acceleration or change of 

 phase on reflexion. This equivalent change of phase is at 

 least double that corresponding to the distance the node is 

 shifted, because of the double journey back and forwards 

 which the reflected wave has to make. But the complete 

 change in phase must be really greater than this, for it is to 

 be remarked that the node observed in these experiments is 

 peculiar, and differs from that ordinarily considered where 

 two non-divergent beams travelling in opposite directions 

 interfere. The beam reflected from a small reflector is di- 

 vergent, and consequently diminishes rapidly in intensity on 

 proceeding outwards. This causes the node or minimum 

 sparking-position to be a little nearer the reflector than the 

 true position of opposite phase. The intensity of the reflected 

 vibration at this point, before it can be subtracted from the 

 incident one, has no doubt to be diminished in proportion to 

 the cosine of the difference in phase between them ; but this 

 is more than made up for by the increase in intensity due to 

 its nearer position, so that the approximation towards com- 

 plete interference is better here than at the true phasial 

 position. That the interference, then, is comparatively slight 

 is seen by the fact that with the smaller sized reflectors it is 



