ELECTRIC WAVES ROUND A PERFECTLY REFLECTING OBSTACLE. 



131 



the distance d of the receiver from the transmitter in rniles, the third column gives 

 the corresponding value of -r) calculated from the formula 



= 2a-r-r cos 



r sn 



the last column gives the ratio of the amplitude F of the oscillations at the receiver 

 to the amplitude F, of the oscillations due to the transmitter alone. 



APPENDIX. INVESTIGATION OF MATHEMATICAL RESULTS REQUIRED. 



1. Summation of Series. 



If u n denotes the general term of a series, and S m the sum of in terms, the first of 

 which is u n , 



which can he written symbolically 



s 2 



