Positive Hay Spectrograph. 713 



field, the bright line with its four fainter companions is due 

 to carbon and its compounds CH, CH 2 , CH 3 , CH 4 . Lines 

 corresponding to still heavier particles are seen as in- 

 distinct patches which would come up in turn to the focal 

 point if the magnetic field were still further increased. 



These results were obtained with residual gas from charcoal, 

 the slits were '05 mm. wide, and the current in the main 

 discharge-tube roughly of the order of one milliampere at 

 30,000 volts. The duration of exposure was 2 minutes in 

 the first three cases and 8 minutes at a rather lower current 

 in the fourth. 



These remarkably short exposures indicate clearly the great 

 advantage obtained by the combined use of a slit system and 

 a focussing arrangement. 



The other parts necessary to complete an apparatus suitable 

 for a general investigation of the relative masses of positive 

 rays are now being constructed, with which it is hoped to 

 obtain results comparable in accuracy with those determined 

 chemically. For masses not too great or widely separated an 

 accuracy of one tenth per cent, is by no means impossible. 

 If anything like this order is obtained, the composition of 

 atmospheric Neon — element or isotopic mixture — will be 

 settled beyond dispute (one of the prime reasons for this 

 work) and several other problems laid open to direct attack. 



It is as well to point out in view of future developments, 

 that second-order corrections in focus are clearly possible by 

 varying the section or even the figure of the poie-pieces 

 and electric plates ; but it is not proposed to employ these 

 refinements until such incidental difficulties as small stray 

 fields due to electrification by the rays themselves have 

 been successfully overcome. 



In conclusion the author wishes to acknowledge his in- 

 debtedness to many friends for their kind advice and help, 

 in particular as regards the mathematical analysis of some of 

 the more complex systems considered which led up to the one 

 finally adopted. Also to the Government Grant Committee 

 of the Royal Society for defraying the cost of parts of the 

 apparatus used in the practical investigation. 



Summary. 



Precision methods of positive ray analysis are discussed 

 and means of improving the brightness of the beam analysed 

 are suggested. 



Theoretical electrical and magnetic analogues of optical 

 cylindrical lenses for focussing positive rays are shown to be 

 possible. 



