614 Dr„ F. W. Aston on tie 



By this arrangement it will be seen that not only is loss of 

 rays by collision and neutralization reduced to a minimum, 

 but any serious leak of gas from the bulb to the camera is 

 eliminated altogether. 



The Electric Field. 



The spreading of the heterogeneous ribbon of rays formed 

 by the slits into an electric spectrum takes place between 

 two parallel flat brass surfaces, J 1? J 2 , 5 cm. long, held 

 2*8 mm. apart by glass distance-pieces, the whole system 

 being wedged immovably in the brass containing-tube 

 in the position shown. The lower surface is cut from a 

 solid cylinder fitting the tube and connected to it and 

 earth. The upper surface is a thick brass plate, which can 

 be raised to the desired potential by means of a set of small 

 storage-cells. In order to have the plates as near together 

 as possible, they are sloped at 1 in 20 — i. e., half the angle 

 of slope of the mean ray of the part of the spectrum which 

 is to be selected by the diaphragms. Of these there are 

 two : one, K lt an oblong aperture in a clean brass plate, is 

 fixed just in front of the second movable one, K 2 , which 

 is mounted in the bore of a carefully ground stopcock L. 

 The function of the first diaphragm is to prevent any 

 possibility of charged rays striking the greasy surface of 

 the plug of the stopcock when the latter is in any working- 

 position. The variable diaphragm is in effect two square 

 apertures sliding past each other as the plug of the stopcock 

 is turned, the fact that they are not in the same plane being 

 irrelevant. When the stopcock is fully open as sketched in 

 fig. 2, the angle of rays passing is a maximum, and may be 

 stopped down to any desired extent by rotation of the plug, 

 becoming zero before any greasy surface is exposed to the 

 rays. Incidentally the stopcock serves another and very 

 convenient use, which is to cut off' the camera from the 

 discharge-tube, so that the latter need not be filled with air 

 each time the former is opened to change the plate. 



The Magnetic Field, 



After leaving the diaphragms the rays pass between the 

 pole-pieces M of a large Du Bois magnet of 2500 turns. 

 The faces of these are circular, 8 cm, diameter, and held 

 3 mm. apart by brass distance-pieces. The cylindrical 

 pole-pieces themselves are soldered into a brass tube 0, 

 which forms part of the camera N. When the latter is 

 built into position, the pole-pieces are drawn by screwed 



