Air-space on Total Reflection of Electric Radiation. 309 



It is seen from, the above, that as the thickness of the air-space 

 was gradually increased, the reflected component increased, while 

 the transmitted portion decreased. The minimum thickness for 

 total reflection was fonnd to be about 8 mm. When the thickness of 

 air-space was reduced to about half this thickness (slightly less than 

 half) the reflected and the transmitted portions seemed to be about 

 equal. 



With the radiator the minimum thickness for total reflection 

 was found to be about equal to the thickness of 22 cards (9*9 mm.). 

 When the thickness of air-space was reduced to the thickness of 10 

 cards (4*5 mm.) the reflected and the transmitted portions seemed to be 

 about equal. As two experiments immediately following each other are 

 more likely to be comparable, the experiments were so arranged that 

 the observation of deflection for transmission with a certain thickness 

 of air followed the observation for reflection with a different thickness, 

 the corresponding deflections being about equal. As stated above, 

 the reflected and the transmitted portions were approximately equal 

 when the thickness of air was equal to the thickness of 10 cards. 

 Keeping 10 as the mean, pairs of readings were taken with different 

 thicknesses. For. example, the reflection reading with a thickness of 

 air equal to the thickness of 4 cards was followed by taking a reading 

 for transmission, with a thickness of air equal to the thickness of 

 16 cards ; the deflections produced in the two cases were about equal, 

 i.e., sixty-six divisions of the scale. 



I append below a table showing the corresponding thicknesses of 

 air (in terms of number of cards) which gave approximately equal 

 deflections, the deflection in one case being due to the reflected com- 

 ponent, and in the other case to the transmitted component. The 

 receiver was made moderately sensitive, so that the deflections lay 

 within the scale. 



Thickness of air for 

 reflection. 



Thickness of air 

 for transmission. 



Deflection producscl. 



4 



16 



66 



6 



14 



70 



— — ■ 



8 



12 



90 j 



10 



10 



120 



When the thickness of air was reduced to 0'45 mm., a deflection of 

 VOL. lxii. • z 



