MASS SPECTRA OF THE ELEMENTS — ASTON. 231 



The method of mounting the cathode will be readily seen from 

 figure 3, which shows part of the apparatus in greater detail. The 

 neck of the bulb is ground off short and cemented with wax to the 

 flat brass collar E, which forms the mouth of an annular space be- 

 tween a wide outer tube F and the inner tube carrying the cathode. 

 The concentric position of the neck is assured by three small ears of 

 brass, not shown. The wax joint is kept cool by circulating water 

 through the copper pipe shown in section at G. 



The gas to be analyzed is admitted from the customary fine leak 

 into the annular space and so to the discharge tube by means of the side 

 tube attached to F, shown in dotted section at Q. Exhaustion is 

 performed by a Gaede mercury pump through a similar tube on the 

 opposite side. The reason for this arrangement is that the space 

 behind the cathode is the only part of the discharge bulb in which 

 the gas is not raised to an extremely high potential. If the inlet or 

 outlet is anywhere in front of the cathode, failing special guards, 

 the discharge is certain to strike to the pump or the gas reservoir. 

 Such special guards have been made in the past by means of dummy 

 cathodes in the bore of the tubes, but, notwithstanding the fact that 

 the gas can only reach the bulb by diffusion, the present arrangement 

 is far more satisfactory and has the additional advantage of enabling 

 the bulb to be dismounted by breaking one joint only. 



THE SLTT SYSTEM. 



The center of the cathode is pierced with a 3-millimeter hole, the 

 back of which is coned out to fit one of the standard slits, S x . The 

 back of the cathode is turned a gas-tight fit in the brass tube 2 centi- 

 meters diameter carrying it, the other end of which bears the brass plug 

 H, which is also coned and fitted with the second slit, S 2 . The two 

 slits, which are 0.05 millimeter wide by 2 millimeters long, can be 

 accurately adjusted parallel by means of their diffraction patterns. 

 The space between the slits, which are about 10 centimeters apart, 

 is kept exhausted to the highest degree by the charcoal tube I v By 

 this arrangement it will be seen that not only is loss of rays by col- 

 lision 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 x J 2 , 5 centimeters long, held 2.8 millimeters apart 

 by glass distance pieces, the whole system being wedged immovably 

 in the brass containing tube in the position shown. The lower sur- 



