Air-space on Total Reflection of Electric Radiation. 303 



radiation striking one face perpendicularly would be transmitted 

 across the opposite face without deviation and cause a response in 

 the receiver. If the cube be now cut across a diagonal, two right- 

 angled isosceles prisms will be obtained. If these two prisms were 

 now separated slightly, keeping the two hypotenuses parallel, the 

 incident radiation would be divided into two portions, of which one 

 portion is transmitted, while the other portion is reflected by the 

 air film in a direction (see fig. 1) at right angles to that of the 



Fig. 1. — Section of the two prisms. 



incident ray, the angle of incidence at the air-space being always 

 45°. The transmitted and the reflected portions would be comple- 

 mentary to each other. When the receiver is placed opposite to the 

 radiator, in the A position, the action on the receiver will be due to 

 the transmitted portion ; but when the receiver is placed at 90°, or 

 in the B position, the action on the receiver will be due to the 

 reflected portion. The advantage of this method is that the two 

 observations for transmission and reflection can be successively taken 

 in a very short time, during which the sensitiveness of the receiver 

 is not likely to undergo any great change. In practice three readings 

 are taken in succession, the first and the third being taken, say, 

 for transmission and the second for reflection. 



I shall now give a general account of the results of the experi- 

 ments. When the prisms are separated by a thickness of air-space 

 greater than the minimum thickness for total reflection, the rays are 

 wholly reflected, there being no response of the receiver in position A, 

 but strong action in position B. As the thickness is gradually 

 decreased below the critical thickness, the rays begin to be trans- 

 mitted. The transmitted portion goes on increasing with the dimi- 

 nution of the thickness of air-space, there being a corresponding 

 diminution of the reflected component of the radiation. When the 

 thickness of the air-space is reduced to about 0'3 mm., no reflected 

 portion can be detected even when the receiver is made extremely 

 sensitive. The reflected component is thus practically reduced to 

 zero, the radiation being now entirely transmitted ; the two prisms, 

 in spite of the breach due to the air-space, are electro-opticaliy con- 



