Eperiments on Diffraction of Cathode Rays. 603 concentrated in a series of spots on the circumference.It should be said at once that there can be no doubt that the whole pattern is due to cathode rays which have been deflected by the film.Thus in the absence of the film only the central spot is seen,and if a magnet is brought up between C and D the whole pattern is shifted together.In some cases,when it is bright enough,this can be seen by following the changes in appearance of the willemite screen when a magnet is brought near.In others,when the rings could not be seen on the screen,photographs were taken with a magnetic field between C and D strong enough to move the central spot,which could always be seen,through a con- siderable distance.In all cases the pattern showed the spot at the centre of the rings,showing that spot and rings had been deflected together.They thus are due to cathode rays of appreciably the same velocity. 6.Variation with the Energy of the Rays.-If the rings are in any way of the nature of a diffraction pattern their size,for a given film,must vary inversely as the wave-length of the waves concerned.According to the formula of de Broglie =hv1-v2/c/mov.If vfc is small =h/mov and if P is the potential difference in volts through which the rays have fallen to acquire their energy,=V2eP/300mo Thus is proportionaltoVPand if D isthe diameter of any given ring DVP should be constant.If the accurate formula is used and allowance made for the variation of mass with velocity in the acceleration of the ray,it is easy to show that=150/ePmo/(1+eP/1200moc2).The correcting term never exceeds 3 per cent.in these experiments.The following table shows the constancy for various films of DVP(1+Pe/1200mc). About equally good agreement is shown by the simple expression DP. Aluminium. Plate. 8.G. P. Dt D1√E(1+P/1200mo). October 7 (3 含 17.500 31 415 October 10 e 9-5 30,500 245 434 Oetober 7 10 31,800 232 418 Oetober 7 (4) 13 40,000 2-12 430 October 7 (5 -+k=t+=n44+ 145 44,000 2-08 445 October 7 165 48.600 1-90 430 October 11 (1) 16-5 48,600 1-98 446 October 12 e 20 66,500 1-83 446 October 12 年年年 2 56,500 1.80 438 Mean 434