272 Dr. H. A. Wilson. [June 3, 



Thus with a lens 100 mm. focal length, aperture ratio F. 7, field 50, 

 the greatest diameter of the image of any point (defect due to i) is 

 0-2 mm. approx. ; that of a point on the axis (defect due to e) will be 

 approx. 0'02 mm. 



For these values the effect of terms in T of higher order would be 

 appreciable ; but the results justify the practice of correcting a single 

 component the back one for astigmatism and spherical aberration, 

 provided due attention is paid to the securing of the condition for no 

 distortion. 



" On the Discharge of Electricity from Hot Platinum." By 

 HAROLD A. WILSON, D.Sc., B.A., Fellow of Trinity College, 

 Cambridge. Communicated by C. T. K. WILSON, F.R.S. 

 Received June 3, Read June 18, 1903. 



(Abstract.) 



This paper contains an account of a series of experiments on the 

 discharge of electricity from hot platinum wires. The main object of 

 the investigation was to determine the influence exerted by the nature 

 of the gas in which the wire is immersed. The first part of the paper 

 contains a short account of some of the results obtained by previous 

 investigators. The rest of the paper is divided into the following 

 sections : 



(1) Description of apparatus, &c. 



(2) The leak in air, nitrogen and water vapour. 



(3) The variation of the negative leak with the temperature. 



(4) The leak in hydrogen. 



(5) The leak from palladium in hydrogen. 



(6) Summary of principal results. 



(7) Conclusion. 



The wire used was of pure platinum, and was mounted like the 

 filament of an incandescent lamp, in a glass tube. A platinum cylinder 

 surrounded the wire, and the current from the wire to the cylinder, with 

 various differences of potential between them, was measured with a 

 galvanometer. The wire was heated by passing a current through it, 

 and its temperature was determined from its resistance. 



It was found that at low pressures using a wire not specially cleaned 

 a large negative leak could be obtained. This leak, however, was not 

 the same on different occasions with the same wire, nor with different 

 wires at the same temperature. The leak on first heating a wire is very 

 large, but falls off with the time. If the wire is then left cold for some 

 hours the leak is again large on first heating and falls off as before. If 

 the wire is kept at a constant temperature and the leak measured for 



