Air-space on Total Reflection of Electric Radiation, 301 



single semi-cylinder of the given substance ; the electric ray was 

 refracted from the denser medium into air, and at the critical angle 

 of incidence it underwent total reflection. The experiment was 

 repeated with two semi- cylinders separated by a parallel air-space. 

 With light waves an extremely thin air-film is effective in producing 

 total reflection. But a question might arise whether waves a hundred 

 thousand times as long would be totally reflected by films of air, and, 

 if so, it would be interesting to find out the minimum thickness of 

 air-space which would be effective in producing this result. This 

 point was raised by Professor Lodge, at the discussion on my paper 

 " On a Complete Apparatus for the Study of the Properties of Electric 

 Waves," read before the Liverpool meeting of the British Associa- 

 tion last year. I have for some time past been engaged in an investi- 

 gation on this subject. The factors which are likely to determine 

 the effective thickness of air-space for total reflection are : (1) the 

 index of refraction of the refracting substance ; (2) the angle of 

 incidence ; (3) the wave-length of the incident electric radiation. 

 In the following investigation, I have studied the influence of the 

 angle of incidence and of the wave-length in modifying the thickness 

 of the effective air-space. The refracting substance used was glass. 



I. Influence of the Angle of Incidence. 



The great experimental difficulty in these investigations lies in the 

 fact, that there is at present no receiver for electric radiation which is 

 very sensitive, and at the same time strictly metrical in its indications. 

 This difficulty is further complicated by the fact that the intensity 

 of the electric radiation cannot be maintained absolutely constant. 

 For these reasons, it is extremely difficult to compare the results 

 obtained from different sets of observations. Attempts have been, 

 made in the following experiments to remove, to a certain extent, 

 some of these difficulties. 



Two semi-cylinders of glass, with a radius of 12*5 cm., were placed 

 on the spectrometer circle. The plane faces were separated by a 

 parallel air-space. The radiator was placed at the principal focus of 

 one of the semi-cylinders ; the rays emerged into the air-space as a 

 parallel beam, and were focussed by the second semi-cylinder on the 

 receiver placed opposite the radiator. Electric radiation was pro- 

 duced by oscillatory discharge between two small circular plates 

 1*2 cm. in diameter and an interposed platinum ball 0*97 cm. in 

 diameter. 



The two semi- cylinders were separated by an air-space 2 cm. in 

 thickness ; this thickness was found to be more than sufficient for 

 total reflection. The critical angle for glass I found to be 2y°. 1 

 commenced my experiments with an angle of incidence of 30° 



