Attenuation of Electric Waves along Wires. 145 



other, or if, as Mr. Blakesley suggests, the two possible 

 positions of the combination for fixed positions of the object 

 and image be taken, and I be the distance through which the 



combination is moved, the focal length is —r, — 7 . 



m 1 — 1 



But this method of determining focal lengths has already 

 been used for several years by Abbe for his optical com- 

 binations, and apparatus for the purpose is to be found 

 described by S. Czapski (Zeitsclirift fur Instrumentenkunde, 

 vol. xii. 1892). A discussion of the method and some account 

 of the apparatus will be found in the very valuable treatise on 

 " Optik," by Czapski, lately published as part of the Handbuch 

 der Physik, just completed under the editorship of Dr. A. 

 Winkelmann (see vol. ii. pp. 289, 290, et seq.). 



Though the experimental method of Mr. Blakesley's paper 

 has thus been in the main anticipated, his paper is valuable as 

 a fresh and instructive view of the subject, and as directing 

 attention to methods of focometry as yet apparently not very 

 generally known. 

 Bangor, June 17, 1897. 



XXII. Attenuation of Electric Waves along Wires and their 

 Reflexion at the Oscillator. By Edwin H. Bakton, D.Sc, 

 F.R.S.E., Senior Lecturer in Pliysics at University College, 

 Nottingliam *. 



LAST year Mr. Bryan and the present author realized 

 experimentally the absorption by a terminal bridge of 

 the electric waves incident upon it f. The first application 

 of this result which suggested itself was the use of such a 

 bridge in determining the attenuation (or rate of decay) of the 

 waves along their parallel leads. For, if one can at pleasure 

 either (1) practically absorb the waves, or (2) reflect them 

 practically undiminished, a suitable electrometer being inserted 

 in the line, we have in the first case a single passage of the 

 wave-train past the electrometer, and in the second a number of 

 passages of the wave-train until it is practically quenched by 

 its repeated losses. Thus, the electrometer in the second 

 case is affected by the sum to infinity of two geometrical 

 progressions, viz., those due to the forward and return 

 waves respectively. Whereas in the first case, when the 

 waves are absorbed at the end of the line, the electrometer 

 is affected by the first term only of one of the above series. 

 Hence the ratio of the electrometer-throws in the two cases is 



* Communicated by the Physical Society : read June 25, 1897. 



t Proc. Phys. Soc. vol. xv. pp. 23-30, 1897 ; Phil. Mag. [5] xliii. 

 pp. 39-45, 1897. 



