232 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



face 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 dia- 

 phragms. Of these there are two : One, K 15 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 

 figure 3, 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 DuBois magnet of 2,500 turns. The faces of these are 

 circular, 8 centimeters diameter, and held 3 millimeters apart by 

 brass distance pieces. The cylindrical pole pieces themselves are 

 soldered into a brass tube O, which forms part of the camera N. 

 When the latter is built into position the pole pieces are drawn by 

 screwed bolts into the arms of the magnet, and so form a structure 

 of great weight and rigidity and provide an admirable foundation 

 for the whole apparatus. Current for the magnet is provided by a 

 special set of large accumulators. The hydrogen lines are brought 

 onto the plate at about 0.2 ampere, and an increase to 5 amperes, 

 which gives practical saturation, only just brings the singly charged 

 mercury lines into view. The discharge is protected from the strong 

 field of the magnet by the usual soft iron plates, not shown. 



THE CAMERA. 



The main body of the camera N is made of stout brass tube 6.4 

 centimeters diameter, shaped to fit onto the transverse tube O con- 

 taining the pole pieces. The construction of the plate holder is in- 

 dicated by the side view in figure 2 and an end-on view in figure 4. 



