608 G.P.Thomson. transparent of these were mounted and tested.The thinnest showed a very well- marked inner ring*(fig.6,Plate 19)and a faint outer one (not reproducible) of about twice the size.The thicker film only showed the inner ring,and that rather faintly,but the size was the same.The inner ring of the thinner specimen varied in size with the voltage in the way now familiar,as shown in the following table:- Spark gap. Voltage. Diameter. DvV. mm. mm. ·3-25 12,000 185 203 45 16,100 15-8 200 7-0 23.200 12-9 196 0·25 29.600 11.9 205 13-0 39.900 100 200 14·75 44,400 97 204 Mean 201 The absolute size of the ring is,however,exactly half that to be expected from platinum taken as a face-centred cube of side 3.91 10-8.It is almost exactly that of the celluloid ring though the latter is rather sharper.It seems possible that these films were really due to some greasy substance present on the bulb (which was originally used for quite a different purpose).A film of this sub- stance would be selected as apparently the thinnest owing to its transparency. Attempts were made to obtain films of undoubted platinum by the same process, but they were unsuccessful as the films always broke while drying on the mount. This increases the probability that the films in question were not platinum. It is difficult to prepare thin platinum films by the methods that has been successful with gold and aluminium as I have been unable to obtain sufficiently thin foil to start with. Conclusions. The detailed agreement shown in these experiments with the de Broglie theory must,I think,be regarded as strong evidence in its favour.These means accepting the view that ordinary Newtonian mechanics(including the relativity modifications)are only a first approximation to the truth,bearing the same In this case the solarisation near the undeflected beam gives the appearance of an extra inner ring.A similar effect,less marked,can be seen in figs.3,4 and 5.These false rings can easily be distinguished in practice by taking photographs with varying exposures. Note added in Proof.-I have since been able to show that platinum films give the rings to be expected from the known crystal structure of the metal,see Nature,' December 3,p.802