23(> 



NATURE 



[May 23, 1918 



screens. I have myself {Fhys. Rev., vol. ii. [1913J, 

 p. 173) subjected this relation to a very precise test and 

 found it to hold accurately. Furthermore, this sort 

 of independence has also been established for the nega- 

 tive electrons emitted by both X- and 7 rays. 



Facts of this sort are evidently difficult to account 

 for on any sort of a spreading-wave theory. But it 

 will be seen that they lend themselves to easy interpre- 

 tation in terms of a corpuscular theory, for if the 

 energy of an escaping electron comes from the absorp- 

 tion of a light-corpuscle, then the energy of emission 

 of the ejected electron ought to be independent of the 

 distance of the source, as it is found to be, and, further- 

 more, corpuscular rays would hit but a very minute 

 fraction of the atoms contained in the space traversed 

 by them. This would explain, then, both the independ- 

 ence of the energy of emission upon intensity and the 

 smallness of the number of atoms ionised. 



In view, however, of the four sets of facts mentioned 

 above, Thomson found it altogether impossible to go 

 back to the old and exploded form of corpuscular 

 theory for an explanation of the new facts as to the 

 emission of electrons under the influence of aether 

 waves. He accordingly attempted to reconcile these 

 troublesome new facts with the wave theory by assum- 

 ing a fibrous structure In the aether and picturing all 

 electromagnetic energy as travelling along Faraday 

 lines of force conceived of as actual strings extending 

 through all space. Although this concept, which we 

 shall call the sether-string theory, is like the corpuscu- 

 lar theory in that the energy, after it leaves the emit- 

 ting body, remains localised in space, and, when ab- 

 sorbed, is absorbed as a whole, yet it is after all essen- 

 tially an aether theory. For in it the speed of propaga- 

 tion is determined by the properties of the medium and 

 has nothing to do with the nature or condition of the 

 source. Thus the last three of the fatal objections to a 

 corpuscular theory are not here encountered. As to 

 the first one, no one has yet shown that Thomson's 

 suggestion is reconcilable with the facts of interfer- 

 ence, though, so far as I know, neither has its irre- 

 concilability been as yet absolutely demonstrated. 



But interference aside, all is not simple and easy for 

 Thomson's theory. For one encounters serious diflfi- 

 culties when he attempts to visualise the universe as 

 an infinite cobweb the threads of which never become 

 tangled or broken,, however swiftly the electrical 

 charges to which they are attached may be flying about. 



Yet the boldness and the difficulties of Thomson's 

 " aether-string " theory did not deter Einstein {Ann. d. 

 Phys. [4], vol. xvii. [1905], p. 132 ; vol. xx. [1906], 

 p. 199) in 1905 from making it even more radical. 

 In order to connect up with some results to which 

 Planck, of Berlin, had been led in studying the facts 

 of black-body radiation, Einstein assumed not only 

 that the energy emitted by any radiator kept together 

 in bunches or quanta as it travelled through space, as 

 Thomson had assumed it to do, but also that a given 

 source could emit and absorb radiant energy only in 

 units which are all exactly equal to ^v, v being the 

 natural frequency of the emitter and h a constant which 

 is the same for all emitters. 



I shall not attempt to present the basis for such an 

 assumption, for, as a matter of fact, it had almost 

 none at the time. But whatever its basis, It enabled 

 Einstein to predict at once that the energy of emission 

 of corpuscles under the influence of light would be 

 governed by the equation 



^mv- = Ve = hv- 



•(4O 



the assumption that it was the whole energy contained 

 in that quantum, p is the work necessary to get the 

 electron out of the metal, and ^mv''^ is the energy with 

 which it leaves the surface — an energy evidently 

 measured by the product of its charge e by the poten- 

 tial difference V, against which it is just able to drive 

 Itself before being brought to rest. 



At the time at which It was made this prediction was 

 as bold as the hypothesis which suggested it, for at 

 that time there were available no experiments whatever 

 for determining anything about how the positive poten- 

 tial V necessary to apply to the illuminated electrode 

 to stop the discharge of negative electrons from it under , 

 the influence of monochromatic light varied with the jl 

 frequency v of the light, or whether the quantity h to s 

 which Planck had already assigned a numerical value W 

 appeared at all In connection with photo-electric dis- 1 

 charge. We are confronted, however, by the astonish- ^ 

 ing situation that after ten years of work at the Ryer- 

 son Laboratory and elsewhere in the discharge of elec- 

 trons by light this equation of Einstein's seems to us 

 to predict accurately all the facts which have been 

 observed. 



The method which has been adopted in the Ryerson 

 Laboratory for testing the correctness of Einstein's 



in which hv is the energy absorbed by the electron 

 from the light wave or light quantum, for according to 

 NO. 2534, VOL. lOl] 



-Photograph of apparatus used for the photo-electric determinatio 

 of Planck's A. 



\ equation has involved the performance of so many 



i op«rations upon the highly inflammable alkali metals 



in a vessel which was freed from the presence of all 



; gases that it is not Inappropriate to describe the present 



experimental arrangement as a machine-shop in vacuo. 



I Fig. 3 shows a photograph of the apparatus, and 



Fig. 4 is a drawing of a section which should make 



the necessary operations intelligible. 



One of the most vital assertions made In Einstein's 

 theory is that the kinetic energy with which mono- 

 chromatic light ejects electrons from any metal is pro- 

 portional to the frequency of the light, i.e. if violet light 

 is of half the wave-length of red light, then the violet 

 light should throw out the electron with twice the 

 energy imparted to It by the red light. In order to 

 test whether any such linear relation exists between 

 the energy of the escaping electron and the light which 

 throws It out it was necessary to use as wide a range 

 of frequencies as possible. This made it necessary to 

 I use the alkali metals, sodium potassium, and lithium, 

 I for electrons are thrown from the ordinary metals only 

 I by ultra-violet light, while the alkali metals respond 



