Page:Collected Physical Papers.djvu/68

48 Observations were now taken to determine the minimum thickness of air for total reflection for different wave-lengths, the angle of incidence being in all cases kept at 45°. Three radiators, R1, R2, R3, were used. I have not yet made determinations of the lengths of wave emitted by these radiators, but it will be seen from the dimensions of the radiators that the waves emitted by R1 are the longest and those emitted by R3 the shortest. The oscillatory discharge in R1 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 R2, the discharge took place between two beads of platinum and an interposed sphere the same as in R1. The distance between the sparking surfaces was 1·01 cm.

In the radiator R3, sparking took place between two beads and an interposed sphere 0·61 cm. in diameter. The distance between the sparking surfaces was 0·76 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, 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.