hertz's researches on electrical waves. 199 



In this way tlie iudex of refraction for the radiatiou of the period era- 

 ployed was found to be about 1.8, so that theparaffine walls which had 

 beeu used were too thick, the proper thickness being about 10 and 20 

 centimeters — exactly so for an incident angle of 51°. On making this 

 alteration I fancied I could detect a slight difference between the re- 

 flections from the tliick and thinner walls; still the difference was 

 not sufficient to be at all satisfactory. The nature of the observing 

 apparatus makes it almost impossible to say if tlie reflection on one oc- 

 casion is more intense or less so than on another so long as si)arks can 

 be obtained. This is due to the sparking point in the rei^eiving appa- 

 ratus continually requiring re-adjustment when working with small 

 sparks, as the distance between them changes either from sliaking or 

 from the points getting burnt up.* Dust, and moisture from the ob- 

 server's breath, are also troublesome. Thus it might be quite possible 

 that the points had always to be much closer with the 20-centimeter 

 wall than with the 10-centimeter wall in order to get sparks, and yet 

 the difference escape detection ; the thing observed being whether 

 sparks can be obtained or not, theeye being incapable of comparing with 

 any degree of accuracy the intensity of light on one occasion with that 

 on another. 



However, if it had been possible to suddenly change the wall, while 

 viewing the sparking, from being 10 to 20 centimeters, it would have 

 been easy to detect any difference which might have existed, but un- 

 fortunately it took some little time to alter the wall. 



In order to obviate this difficulty the following device was resorted 

 to with the object of showing that there was a difference in the be- 

 havior of the wall when 10 centimeters thick to its behavior wlien 20 

 centimeters thick. (For at the time I did not see that the experiment 

 was inconclusive, the effects observed being the same whether tlie back 

 reflected at all or not.) A small sheet of zinc was placed at the back 

 of the wall, and the effect on the sparking- observed while an attend- 

 ant suddenl}^ removed or again replaced the zinc. It was supposed 

 that when the wall was 20 centimeters thick, and there was sparking, 

 that on suddenly placing the zinc on the back the sparking would in- 

 crease, owing to the phase of the reflection from the back being half 

 a period different from that of the reflection from the zinc ; but when 

 the wall was 10 centimeters thick that the presence of the zinc would 

 diminish the sparking. 



It was with no little surprise that the reverse was observed. That 

 is to say, i^lacing a sheet of zinc about 30 centimeters square on the 

 back of the wall actually aided the reflection from the back so as to 

 diminish the sparking with the 20-centimeter wall, but increasing it 

 with the 10-centiraeter wall. Tiiis observation made it look as if it 

 must be on the first reflection from the paraffine (that is to say, on 



* With very small sparka the tberinal expansion must be counteracted by unscrew- 

 ing. 



