Air-space on Total Reflection of Electric Radiation. 305 



Observations "were now taken to determine the minimum thickness 

 of air for total reflection for different wave-lengths, the angle of inci- 

 dence being in all cases kept at 45°. Three radiators, R l5 R 2 , K-3» 

 were used. I have not yet made determinations of the lengths of 

 wave emitted by these radiators, but it will be seen from the dimen- 

 sions of the radiators that the waves emitted by Rx are the longest 

 and those emitted by R 3 the shortest. The oscillatory discharge in 

 Ri took place between two circular plates 1'2 cm. in diameter and an 

 interposed ball of platinum 0'97 cm. in diameter. The radiators 

 were enclosed in a tube 3'8 cm. in diameter. 



In the radiator R 2 , the discharge took place between two beads of 

 platinum and an interposed sphere the same as in R x . The distance 

 between the sparking surfaces was 1*01 cm. 



In the radiator R 3 , sparking took place between two beads and an 

 interposed sphere 0'61 cm. in diameter. The distance between the 

 sparking surfaces was 76 cm. 



One prism was fixed on the spectrometer circle, and the other was 

 at first placed somewhat apart from it; the distance was now 

 gradually reduced till the air-space just ceased to reflect totally 5 

 when a small portion of radiation began to be transmitted. The 

 beginning of transmission was detected by the receiver, which was 

 placed in the A position. The detection of the beginning of trans- 

 mission is, as has been said before, somewhat dependent on the 

 sensitiveness of the receiver. 



Radiator. 



Distance between spark- 

 ing surfaces in mm. 



Minimum thickness for total 

 reflection. 





R-i 







(a) 



^2 



10 -1 





(b) 





7-6 





(c) 



From the above results it is seen that the effective thickness of the 

 totally reflecting air-space increases with the wave-length. If the 

 wave-lengths are proportional to the distance between the sparking 

 surfaces which give rise to the oscillatory discharge, the wave-lengths 

 in (b) and (c) are in the ratio of 101 : 76. This is not very different 

 from the ratio of the corresponding minimum thicknesses of the totally 

 reflecting air-space. 



III. On the Relation betiveen the Reflected and the Transmitted Compo- 

 nents of Radiation when the Thickness of Air-space undergoes 

 Variation. 



In the general account of the experiments, I have said that as the 

 thickness of air-space is gradually reduced the intensity of the 



