1398 - 99 .] Eev. A. Moffat on Energy of the Eontgen Bays. 431 
"was driven by a hydraulic motor made by A. Schmid of Zurich. 
I regulated the water supply so that the motor ran at the rate of 
A = Amyl acetate lamp ; B = Box to screen off light ; C= Aperture in B ; 
D = Optical bank; E = Photometer ; F = Barium platino-cyanide screen; 
G = Rontgen lamp ; H = Spark interrupter ; J — Box to screen off light ; K = 
Shunt ; L = Tube from bellows ; M = Bellows ; FT = Influence machine ; 0 = 
Motor to drive it. 
two revolutions per second. In order to obtain as strong Eontgen 
rays as possible, I introduced an air-spark into the circuit. It has 
been shown by Dr A. Wehnelt and others that a Eontgen lamp 
is the more efficient the more abruptly the discharge takes place, 
and I found this to be the case in my experiments. The dis- 
charge in the air-spark was made as disruptive as possible by blow T - 
ing a strong current of air through it. This drives away the 
particles of dust in the air, and consequently makes the discharge 
more sudden. 
The Eontgen lamp, with its air-spark, was enclosed in a box with 
a wooden frame and sides of thick cardboard. The lamp was fixed 
near one side of the box, and at the place where the rays from the 
anti-cathode were to be transmitted a hole 6 cm. in diameter was 
bored and covered with thin black paper. On the other side of 
this thin black paper the barium platino-cyanide screen was 
fastened. In front of the screen was fixed a piece of cardboard 
