296 Dr. E. H. Barton on the Attenuation of Electric 



reproduction. It may be of interest to refer to some other 

 experiments which have been tried with success, and also to 

 record some of the many failures. 



Using a fork of frequency 220 the mercury was disturbed 

 by two whole-period point combs moving in opposite phase. 

 They produced a focus, which, from a calculation like that for 

 fig. 9, Paper 2, was found to be in a position in accord with 

 theory. The optical analogue of this experiment is found in 

 the phase-reversal zone-plates of Mr. R. W. Wood*. The 

 experiment was suggested by a note in Preston's ' Light/ 

 p. 218, 1895 edition. 



By using a comb like that in fig. 9, it might be possible to 

 produce the analogue of spectrum analysis. Experiments in 

 this direction have been unsuccessful. Using the grating for 

 reflexion, only faint traces of the effect have been obtained. 

 In another experiment a point was actuated by two simul- 

 taneous simple harmonic motions in a vertical direction. The 

 frequencies were chosen so as to give four beat-lines on the 

 negative ; these are distinctly visible, the ripples being w r ell- 

 defined between the beat -lines. 



Another experiment which has met with success was con- 

 ducted with a circular reflector The reflector was an accu- 

 rately turned, light annulus of ebonite, made as flat as 

 possible by warming between steel true planes. The point 

 source bisected a radius of the enclosed circle. The pattern, 

 as obtained by geometry, is given in Weber's Wellenlehre, 

 figs. 52, 53. The photograph is in accordance with theory. 



XXVIII. Attenuation of Electric Waves along a Line of 

 Negligible Leakage. By Edwin H. Barton, D.Sc, 

 F.R.S.E., Senior Lecturer in Physics, University College, 

 Nottingham f. 



SHORTLY after the publication of my previous paper % 

 on this subject Mr. Oliver Heaviside pointed out to me 

 that it would be interesting to compare the observed attenua- 

 tion in the transit of the waves with that to be expected on 

 the long-wave theory. Be was also good enough to draw 

 my attention to Lord Rayleigh's high-frequency formula for 

 the effective resistance of wires to alternating currents as 

 approximately applicable to the case. He feared, however, 



* Phil. Mag. Jane 1898. 

 , t Communicated by the Physical Society : read June 10, 1898. 

 S T X Phil. Mag. August 1897 ; Proc. Phys. Soc. Dec. 1897, Jan. 1898. 



