to the range of wavelengths shown by a Fourier analysis of the train. But the motion of the shutter might well be expected to alter the velocity of particle passing through, Just before it clased. this initi al motion wit hout astur bi ng it ho pelessly. There seems no reas on why those w ho prefer it s houl motion of particles governed by laws which sim ulate the character of waves, but besides the rat her arti ficial character of the law of motion, one has to ascribe importance to the detailed initial conditions of the motion which, as far as our present knowledge goes, are necessar ily incapable of being determined. I am predisposed by nature in favour of the most mechanical explanations possible, but I feel that this view is rather clumsy and that it might be best, as it is certai nly safer, to keep strictly to the facts and re gar d the wave equati on as merely a way of predicting the result of ex perime Nevertheless, the view I have sketched is often a help in thinking of these problems. We are curiously near the position whic h Newton an that of his rivals an Those that are averse from assenting to any new Discoveries, but such as they can explain by an Hypothesis, may for the pres ent suppose, that as Stones by falling upon w ater put the w ater into an undulati ng Moti on, and all B odes by percussion excite vi brations in the Air: so the Rays of ught, by impinging on any refracting or reflecting Surface, excite vibrations in the refracting or reflecting Medium or Substance, muc h after the manner that vibrations are propagated in the Air for ca using sound, and move taster thant he Rays so as to overtake them; and that when any Ray is in that part of the vibration which conspires with its Motion, it easily breaks through a refracting surface, but when it is in the contrary part of the vibration which impedes its Motion, it is easily reflected; and, by conseque nce, that every Ray is s uccessivel y dispos ed to be easily ref ect ed, or easily transmitted, by ery vibration which overtakes it. But not here consider Although the experiments in diffraction confirm so beautifully the de Broglie-Schr t inger wave theory the position is less satisfactory as regards th On this theory the electron possesses magnetic properties and the wave requires four quantities instead of one for its specification. This satis fies those needs of spectroscopy which led to the inventi on of the spi ng electron. It suggests however that electronic waves could be pol arized and that the polariz ed waves might int eract wit h matter in an anis tropic manner. In fact detailed calculations by Mott indicate th if Dirac electrons of 140 kv energy are scattered twice through 90 by the nuclei of gold atoms the intensity of the scattered beam will dinter by 16% according to whether the two scatterings are in the same or in opposite drections. Experiments by Dymond and by myself have established independently that no eftect of this order of magnitude exists, when the scattering is done by gold foils. while there is a slight possibility that th circumnuclear electrons, or the or ganization of the atoms into crystals might effect the res ut, it seems very unlikely. Some of the theor ists ha ve arrived at results conflicting with Mott nderstand that their wok has been found to contain errors. At present there seems no explanation of this repancy which throws doubt on the validity of the Dirac equations in spite of their success in predicting the positive el ectron. I shoul d be sorry to leave you with the im pression t hat electron di ffraction w as of interest only to those concerned with the fundamentals of physics. It has im portant practical applications to the study of s urface effects. You know how x-ray diffraction has made it possi ble to determine the arrangement of the atoms in a great variety of s olds and even liquids. x-rays are very penetrating, and any structure pec liar to the surface of a body will be likely to be overlooked, for its effect is swamped in that of t he m uch greater mass of underlying material. Electrons only affect lay ers of a few atoms, or at most ens of atoms, in thic kness, and so are emi ne ntly s uited for the pur pose. The position of the beams diffracted from a surface enables us, at least in may cases,to deter mine the arrangeme nt of the atoms in the sur face. Among the many cases which ha ve alre ady been st ud ed I have only time to refer to one, the state of the surface of polished metals. Many years ago sir George Beil by suggested that this resembled a s upercodle d liquid which had flowed under the stress of polishing. A senes of ex periments by electron dif fracti on carried out at the Im perial Coll ege in London has confirm ed this conclusi on The most recent work due to Dr. Coc hrane has shown that though t his am orphous i ayer is sta ble at ordi nary tem perature as long as it remains fixed to recrystalize after a few hours. Work by Professor Finch on these lines has led to valuable conclusions as to the wear on the surtaces o cylinders and pistons in petrol engines. It is in keeping with the unive rsal character of physical science t hat this single small branch of it should touc h on the one hand on the fundam ent als scientific philasophy and on the other, questions of everyday lito the range of wavelengths shown by a Fourier analysis of the train. But the motion of the shutter might well be expected to alter the velocity of a particle passing through, just before it closed. Agai n, on the new view it is purel y a matter o f chance in whic h o f t he di ffr acted beams of di fferent or ders an el ectron appears. I f the phenomenon w ere expr essed as t he classical m otion o f a particle, t his woul d hav e to depend on the i nitial m otion of the particle, and t here is no possi bility of determi ning this initi al m otion wit hout distur bi ng it hopelessly. Ther e seems no reas on why t hose w ho pre fe r it s houl d not regar d t he di ffr action o f electr ons as t he motion o f pa rticles gov erned by l aws w hich sim ulat e t he c har ac ter o f waves, but besides the rat her arti ficial cha racter o f t he l aw of moti on, one has to ascribe importance to the detailed initial conditions of the motion which, as far as our present knowledge goes, are necessar ily incapable of being determined. I am predisposed by nature in favour of the most mechanical explanations possible, but I feel that this view is rather clumsy and that it might be best, as it is certai nly safer, to keep strictly t o the facts and regar d the wave equati on as m erely a w ay of pr edicting t he r esult of experiments. Nevertheless, the view I have sketched is often a help in thinking of these problems. We are curiously near the position whic h Newton took over his theory of optics, long despised but now seen to be far nearer the truth than that of his rivals and successors. "Those that are averse from assenting to any new Discoveries, but such as they can explain by an Hypothesis, may for the pres ent suppose, that as Stones by falling upon w ater put the W ater i nto an undulati ng Moti on, an d all B odies by percussi on excite vi brati ons i n t he Air: so t he Rays of Li ght, by impinging on any refracting or reflecting Surface, excite vibrations in the refracting or reflecting Medium or Substance, muc h after the manner that vibrati ons are pr opagated i n t he Air for causing S ound, and mov e faster than t he Rays so as to ov ertak e t hem; and that when any R ay is in that part o f the vibration which conspires with its Motion, it easily breaks through a refracting Surface, but when it is in the contrary part of the vibration which impedes its Moti on, it is easily re flected; and, by consequence, t hat ev ery Ray is s uccessivel y dispos ed to be easily refl ect ed, or easil y tr ansmitted, by every vibration which overtakes it. But whether this Hypothesis be true or false I do not here consider." Although the experiments in diffraction confirm so beautifully the de Broglie-Schr 鰀 inger wave theory, the position is less satisfactory as regards the extended theory due to Dirac. On this theory the electron possesses magnetic properties and the wave requires four quantities instead of one for its specificati on. This satis fies t hos e needs o f spectroscopy whic h l ed to the i nv enti on o f t he spi nni ng electron. It suggests however that el ectronic waves coul d be pol arized and t hat the polariz ed wav es mi ght int eract wit h matter in an anis otropic manner. I n fact detailed c alculati ons by M ott i ndicat e t hat if Dirac electrons of 140 kV energy are scattered twice through 90° by the nuclei of gold atoms the intensity of the scattered beam will differ by 16% according to whether the two scatterings are in the same or in opposite directions. Experiments by Dymond and by myself have established independently that no effect of this order of magnitude exists, when the scattering is done by gold foils. While there is a slight possibility that the circumnuclea r electr ons, or the or ganizati on of t he atoms i nto cr ystals mi ght effect the res ult, it seems very unlik ely. Som e o f t he t heor ists have a rrived at results conflicting with Mott, but I understand that their work has been found to contain errors. At present there seems no explanation of this discrepancy which throws doubt on the validity of the Dirac equations in spite of their success in predicting the positive el ectron. I shoul d be s orry to l eav e you with the im pression t hat electr on di ffraction w as o f i nter est only t o t hose concer ned with the fundamentals o f physics. I t has im portant pr actical applications t o the st udy o f s urface effects. You know how X- ray di ffraction has m ade it possi ble t o dete rmine t he arrangement of t he at oms i n a great v ariety o f s olids and ev en liqui ds. X-rays a re ver y penetrati ng, and any structure pec uliar to the surface of a body will be lik ely to be overl ooked, for its e ffect is swamped i n that of t he m uch greater m ass of underl ying m aterial. Electrons only a ffect lay ers o f a few atoms, or at most tens o f at oms, i n thic kness, and so a re emi nently s uited for the pur pose. The position o f the beams diffracted from a sur face enables us, at l east in many cases, to determine t he arrangement of the atoms i n t he sur face. A mong the m any cases whic h have already been st udi ed I hav e onl y tim e to re fer to one, t he st ate of t he sur face o f polished metals. M any y ears ago Sir George Beil by suggested t hat t his r esembl ed a s upercoole d liqui d whic h had flowed under the stress o f polishi ng. A series of experim ents by electron diffracti on c arried out at the Im perial Coll ege in London has confirm ed t his conclusi on. The most recent work due t o Dr. Coc hrane has shown that though t his am orphous l aye r is stable at ordi nar y tem pe rature as long as it rem ains fix ed to the mass of the metal, it is unstable when removed, and recrystalizes after a few hours. Work by Professor Finch on these lines has led to valuable conclusions as to the wear on the surfaces of cylinders and pistons in petrol engines. It is in k eepi ng wit h t he unive rsal c har acter of physical science t hat this si ngl e sm all branch o f it shoul d touc h on the one hand on t he fundam ent als o f scientific philosophy and on the other, questions of everyday life