302 Dr. J. C. Bose. On the Influence of the Thickness of 



(slightly greater than the critical angle). The receiver, which was 

 placed opposite the radiator, remained unaffected as long as the rays 

 were totally reflected. But on gradually diminishing the thickness 

 of air-space by bringing the second semi-cylinder nearer the first 

 (always maintaining the plane surfaces of the semi-cylinders parallel), 

 a critical thickness was reached when a small portion of the radia- 

 tion began to be transmitted, the air-space just failing to produce 

 total reflection. The beginning of transmission could easily be 

 detected and the critical thickness of air determined with tolerable 

 accuracy. The slight discrepancy in the different determinations 

 was due to the unavoidable variation of the sensitiveness of the 

 receiver. When the thickness of air was reduced to 14 mm., the 

 receiver began occasionally to be affected, though rather feebly. 

 But when the thickness was reduced to 13 mm. there was no uncer- 

 tainty ; a measurable, though small, portion of the radiation was 

 now found to be always transmitted. 



I now increased the angle of incidence to 45°, and observed that 

 the minimum thickness, which at 30° just allowed a small portion 

 of radiation to be transmitted, was not sufficiently small to allow 

 transmission at the increased angle of incidence. The thickness had 

 to be reduced to something between 103 mm. and 9"9 mm. for the 

 beginning of transmission. 



With an angle of incidence of 60°, the minimum thickness for total 

 reflection was found to be between 7*6 mm. and 7*2 mm. 



Angle of incidence. 



Minimum thickness of air 

 for total reflection. 



30° 



45 



60 



Between 14 and 13 mm. 

 „ 10*3 and 9 9 mm. 

 „ 7"6 and 7"2 mm. 



The minimum effective thickness is thus seen to undergo a 

 diminution with the increase of the angle of incidence. 



II. The influence of the Wave-length. 



In the following experiments I kept the angle of incidence con- 

 stant, and varied the wave-length. I used three different radiators, 

 A, B, and C ; of these A emitted the longest, and the shortest 

 waves. 



The following method of experimenting was adopted as offering 

 some special advantages. If a cube of glass be interposed between 

 the radiator and the receiver placed opposite to each other, the 



