MASS SPECTRA OF THE ELEMENTS — ASTON. 229 



Beferred to axes OX, OY, the focus is at r cos (0—20'), r sin 

 (<t>— 20'), or r, b.20'; so that to a first-order approximation, what- 

 ever the fields, so long as the position of the diaphragm is fixed, 

 the foci will ail lie on the straight line ZF drawn through Z 

 parallel to OX. For purposes of construction G the image of Z in 

 OY is a convenient reference point, 4> being here equal to 40. It 

 is clear that a photographic plate, indicated by the thick line, will 

 be in fair focus for values of e/m over a range large enough for 

 accurate comparison of masses. 



The arrangement, which has a distinct resemblance to the ordi- 

 nary quartz spectrograph, gives very complete control. The field 

 between the plates can be adjusted to allow the brightest part of the 

 electric spectrum to be used which, as has been shown, is in general 

 the same for all normal rays under steady discharge, and the values 

 of e/m can be compared very accurately from the positions of their 

 lines relative to those of standard elements which can be brought 

 to any desired position on the plate by varying the magnetic field 

 strength. 



CONSTRUCTION OP THE MASS SPECTROGRAPH. 

 THE SLITS. 



The very fine slits used in this apparatus were made with com- 

 parative ease, as follows: A cylinder of pure aluminium about 10 

 millimeters long by 5 millimeters wide is carefully bored with a 

 hole 1 millimeter diameter. The resulting thick-walled tube is then 

 cleaned and crushed with a hammer on an anvil until the circular 

 hole becomes a slit about 3 millimeters wide. Continuation of this 

 treatment would result in a slit as fine as required, giving the maxi- 

 mum resistance to the passage of gas, but its great depth would make 

 the lining up of a pair a matter of extreme difficulty. The crushed 

 tube is therefore now placed between two V-shaped pieces of steel 

 and further crushed between the points of the V's at about its middle 

 point until the required fineness is attained. Practice shows that the 

 best way of doing this is to crush until the walls just touch, and then 

 to open the slit to the required width by judicious tapping at right 

 angles to that previously employed. With a little care it is possible 

 to make slits with beautifully parallel sides to almost any degree of 

 fineness, 0.01 millimeter being easily attainable. At this stage the 

 irregularly shaped piece of aluminium is not suited to accurate gas- 

 tight fitting; it is therefore filled with hard paraffin to protect it from 

 small particles of metal, etc., which, if entering, can not be dislodged 

 owing to its shape, and turned up taper to fit the standard mountings. 

 These in the present apparatus are taper holes in the back of the 

 cathode and in a corresponding brass plug at the ends of a wide tube 



