202 hertz's researches on electrical waves. 



ineDt is iuteresyufj in coimectiou with the electro-maguetic way of 

 looking at the acceleration of phase as being dne to the accumulations 

 of electricity on the edges of the reflector, which is the same as the 

 reason why it is necessary to use lo7ig cylindrical mirrors, as was pointed 

 out by Professor Hertz in a letter last February to Professor Fitz- 

 gerald, This experiment is really the same as Stokes's experimentum 

 crucis, as Professor Fitzgerald points out. 



If instead of using the whole primary wave in the former experiment, 

 it be passed through a screen with a hole in it (either square or a 

 long slit at right angles to the electric displacement), the position of 

 interlerence, as might be anticipated, was not shifted out as much as 

 before. In the rough experiment made, it was found to occur at about 

 19 centimeters from the screen. 



It was now thought well to repeat the determination of the index of 

 refraction with a larger wall and metallic reflector than had been used 

 before, as this change of phase might have affected the former re- 

 sults. But it was found that it had not done so to a sensible extent. 

 However, the result of these new experiments was finally to give for 

 parafQn, /< = 1.75, and at the same time it was found that the wave- 

 length given by the " vibrator" was G8 and not 66 centimeters, as had 

 been assumed. 



Two new knobs for the " vibrator '' had been made, and the fact had 

 been overlooked that they were slightly larger than the old ones, which 

 gave a wave-length of 66 centimeters. These new knobs were electro- 

 plated with gold, and were a great saving of trouble, as they could be 

 cleaned by merely rubbing with paper ; apparently, the gold carried 

 across by the sparking (in the form of a black powder) coming off", — ' 

 but some may have re-burnished on. It was a curious thing that if the 

 knobs were left uncleaned over night, the next morning it was very 

 hard to get the black off", — some molecular change probably occur 

 ring. 



If the value of yu thus found be not in some way due to the parafQn 

 being in separate blocks, it would show a remarkable anomalous dis- 

 persion for paraffin near these curiously slow vibrations, and as sug- 

 gested by Professor Fitzgerald, may be connected with the vibration 

 periods of atoms in the molecule, as it can hardly be connected with the 

 vibrations in the atoms themselves. It might be interesting to inves- 

 tigate whether these slow vibrations could cause dissociation, and thus 

 lead to a photographic method of observing them. It may also be 

 allied with ordinary electrolysis by very long period currents, as is 

 also suggested by Professor Fitzgerald. 



Assuming ^/ =1.75,* and A=68 centimeters, the thicknesses of the 

 walls in the " Newton's ring" experiment, as above described, were wrong. 

 However, it was found more convenient to alter the angle of incidence to 



*Thi8 value agrees with polarization experiments. No reflection was obtained at 

 its corresponding angle, while at tan-' 1.51 some sparks were occasionally seen. 



