98 



Prof. J. A. Pollock on the Wave-length of the 



mirror, have also been determined with the same coherer 

 arrangement. The results exactly agree with those found 

 with the interferometer. 



For these and the interferometer measurements, the 

 oscillator, sparking in paraffin, was placed at the focus of 

 a cylindrical parabolic mirror, the focal length being an odd 

 multiple of a quarter of the wave-length to be expected. 

 Experiments have been made with mirrors whose focal 

 lengths were calculated from the lengths of the oscillators 

 on the basis of both Abraham's and Macdonald's theories. 

 Ives' conclusion that the focal length of the oscillator mirror 

 has a negligible effect on the wave-length has been verified. 



The various values which have been found are set out in 

 the following table, together with the names of the experi- 

 menters who carried out the considerable practical work 

 involved in the determinations. To make the evidence 

 complete, I have included the results of Ives' measurements. 

 The other measures, previously mentioned, are not given 

 in the table, as the earlier experiments are open to the 

 criticism that the influence of the receivers was not entirely 

 eliminated. Ives' use of a very high resistance in the 

 receiver circuit quite obviates an error of this kind. In the 

 experiments in this laboratory, coherers of very dissimilar 

 dimensions were employed in verifying the results, and it 

 was definitely proved that the measurements were wholly 

 independent of any characteristic of the receiving circuits. 



Experimenters. 



Length 



of 



Oscillator. 



\ 



obs. 



calc. 



k 

 obs. 



211 



Jc 

 calc. 



2-05 



Experi- 

 mental 

 Method. 



Number 

 of nodes 

 deter- 

 mined. 



Approxi- 

 mate dis- 

 tance from 

 plane 

 mirror. 



i Ives 



4-93 



10-42 



10-12 



Interfero- 

 meter. 











7-49 



15-24 



15-30 



2-03 



2-04 



„ 









9-85 



1986 



20-06 



2-02 



2-04 



» 







2 Booth&Tiddy 



5-06 



10-40 



10-30 



2-06 



206 



)> 



3 







;» 



10-54 



>' 



2-08 



" 



Stationary 

 Waves. 



4 



100 X 



>i » 



>> 



1046 



i) 



2-07 



ii 



,, 



4 



., 



>> j j 



V 



10-40 



» 



206 



ii 



» 



3 



5\ 



?) )> 



18-84 



38-47 



38-38 



205 



203 



„ 



11 



20 X 



3 Anderson & 





38-00 





2-01 







5 



5X 



Scarr. 



















1 Ives, loe. cit. 2 E. H. Booth, H. P. Tiddy. 



3 R. C. Anderson, J. H. A. Scarr. 



