Experiments on Diffraction of Cathode Rays. 607 of introducing n as a separate variable.)The "observed"values are ratios calculated so as to make the diameter of the first ring3. Observed. (伍，) √++区 October 17. October 12 (7). (2,1,1) 3 5 (2,0,0) 在=2-00 205 202 (2,2,0) 8=288 281 283 (1,1,3) /=332 (2,2,2) /12=347 ,3·40mean 3:40 3-33 (4,0,0) √/16=400 absent absent (3,3,1) 19=436 (4,2,0) /20=447 4.41 mean 444 445 It willbe seen that the agreement is very good,that all the rings can be accounted for,and that all the rings to be expected in the region covered by the plate are present,except the higher order of the(1,0,0)which might be expected to be very faint.The rings V1l and√12，and again√19and√2o0 are not resolved. As regards the absolute value of“a”I find a=3·80×l0-8.TheX-ray result is a=4.065 x 10-8,a discrepancy of 6 per cent.,to which the same remarks apply as in the case of aluminium. 10.Celluloid.-A few photographs were taken with a thin film of celluloid, of the order of 5 x 10-6 cm.thick,similar to those used by Mr.Reid in the experiments described in'Nature.'One of these is shown in fig.5,Plate 19. There is a well-defined inner ring and a much fainter outer one of about twice the size not visible in the reproduction.Mr.Reid is continuing the work on these films with the apparatus originally used,which,while slightly more complicated,has the advantage that the energy of the rays can be measured with much greater accuracy.It is hoped that a detailed account of this work will shortly be ready for publication. Film X.-The first experiments with the apparatus described were made with a thin film believed to be platinum.This was obtained by acting with aqua regia on the film formed on the inside of a bulb by cathode spluttering from platinum.It was found accidentally in the course of cleaning the bulb that the film came off in the acid in flakes 2 or 3 mm.across.Two of the most 2T2