placed opposite to each other, the 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 out 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. 9) at right angles to that of the incident ray, the angle of incidence at the
Fig. 9. Section of the two prisms.
air-space being always 45°. The transmitted and the reflected components would be complementary 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 within a very short time, during which the sensitiveness of the receiver is not likely to undergo any appreciable change. In practice three readings are taken in succession, the first and the third being taken, say, for transmission and the second for reflection.
