(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 radiation 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 uncertainty; 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 10·3 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 constant, and varied the wave-length. I used three different radiators, A, B, and C; of these A emitted the longest, and C 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
