The interpretation of optical interference experiments at low light intensities and with independent light sources is discussed. A new experiment on interference between modulated light beams is described briefly, and its significance is indicated. There are two appendices, one a fuller discussion of the conditions under which interference and intensity correlation may be produced, the other a comment on one aspect of Jánossy and Nagy's discussion (1956) of the feasibility of the alternating slit interference experiment.

In this paper we discuss the structure semigroup of the L1-algebra of an Arens-Singer semigroup. Arising from this study we provide a complete and rather unexpected description of a nontrivial structure semigroup. We then link the above ideas with that of the almost periodic compactification of the semigroup.

For comfort the total future world population faces a limit. Three or more educated generations seem likely before the population growth will be checked. It seems prudent to envisage a world population of about 15 thousand million in a century or two. Their energy needs for food, water, warmth, clean air, locomotion and management of wastes may be assessed at 5 to 50 thermal kilowatts per capita. That amounts to 0·05 per cent, to 0·5 per cent, of the energy supply from the sun. Generating this amount by nuclear fission would cause no severe pressure on world resources of uranium and thorium for hundreds of centuries, if near-breeders such as CANDU reactors are used exploiting the uranium-thorium fuel cycle. Neither the cost of fuel supply nor of waste management to keep the environment unburdened by extra radioactivity need be as high as prices now paid for coal and oil. The addition of breeders or other means such as nuclear fusion or spallation for increasing the neutron supply would significantly lower the use of uranium and thorium but would not necessarily reduce the cost of power.

One of the traditional activities of the National Physical Laboratory is its work on the maintenance and improvement of the primary standards of measurement. Although one may possibly visualise such work, because of its long history and its association with calibration services, as of a largely routine character, this is certainly far from the case at the present time. The present is a period of considerable activity and change in fundamental metrology, with the classical material standards of measurement being superseded by atomic or quantum standards. The past decade has seen a change to atomic standards for the units of length and time, and there seems little doubt that the future will see an extension of atomic-based standards to other areas, notably that of the electrical quantities. Some of the changes which may come about as a consequence of adopting the most accurate and convenient quantum methods have interesting implications. For example, a possible outcome of the new techniques being developed for the accurate measurement of very high (infrared) frequencies is that the standards of length and time may become unified, with the velocity of light taking the role of an agreed defined constant rather than an experimentally determinable quantity.

The problem of the simultaneous transport of solute and water through a cell membrane is examined from a kinetic standpoint. It is shown that the results obtained by this approach are in accord with those obtained conventionally by irreversible thermodynamics. Stress is laid on the fact that the kinetic theory has certain advantages over the conventional approach in that it provides, among other things, a form of membrane model and at the same time its range of validity is indicated.

The elastic properties of plant cell walls are major factors controlling the swelling and shrinking of plant cells under conditions of varying water stress. The importance of these properties for the adaptation of plants to the changing water stresses they encounter is discussed.

The space group for paraelectric KH2PO4 is described and tables of irreducible representations are presented. These are used to derive the symmetry of the vibrational modes for both the heavy atoms and the hydrogen atoms, the latter being treated by a simple pseudo-spin model. Recent experiments on KH2PO4 and related materials are analysed in terms of the mode symmetries, and the light these experiments throw upon the nature of the phase transitions is briefly discussed.

A search was made for branching to the 1–84 MeV state of 42Ca in the 42Sc ground state decay. This was found to be less than 0·02 per cent. The branching has a negligible effect on the calculation of the vector coupling constant Gv for the weak interaction from the 42Sc decay.

The existence of a work by Aristotle on the constitution of Athens is attested by the historians Philochorus, writing before 306 B.C., and Timaeus, writing about 300 B.C. No trace of such a work was known until 1891, when the almost complete text of one was recovered from the sands of Egypt by F. G. Kenyon of the British Museum. It was then recognised that two small and much damaged sheets found ten years before also contained parts of this work.

The existence and the properties of the solutions of a nonlinear differential system describing a diffusion system with concentration-dependent diffusion coefficients are investigated. It is shown that there exists at least one solution which is monotonie decreasing and asymptotically tends to the prescribed value.

It is shown that, from an operational point of view, the production and detection of spin-polarised electrons by Mott scattering is equivalent to the performance of a Stern-Gerlach type experiment with heavy spin-one-half particles. The argument is based on standard quantum mechanical formalism using density matrices and Stokes vectors for the description of polarised assemblies of particles.

A prototype source of polarised electrons is described, in which electrons trapped by a combination of electric and magnetic fields are polarised through spin exchange interaction with a polarised beam of potassium atoms. Under typical operating conditions, a pulsed electron beam was extracted with 104 electrons per pulse and a polarisation of 0·45 at a repetition rate of 50 S−1. The scope for improvement of these figures is briefly discussed.

Values for the total spin exchange collision cross-section at electron energies of 1 eV, 2·5 eV and 4 eV were obtained. These are in reasonable agreement with theoretical predictions.

The temperature dependence of the properties of magnetic crystals containing defects is calculated with a simple linear chain model. The model enables the spin deviations caused by the impurity to be treated throughout the whole crystal instead of being restricted to the neighbourhood of the defect as in other recent theories. The results are in qualitative agreement with experimental results and suggest that stable spin clusters may be found surrounding each defect above Tc. The approximations used in other theories are shown to be suspect above 0·5 Tc.

The author maintains that a course in the classical theory of the electromagnetic field, with full exploitation of vector calculus methods, should be thought of as being as much of a basic essential in any physics honours course as is a course on classical mechanics. It is suggested that if the mathematical techniques are taught in a way that relates them directly to the central notions of field theory and avoids discussion of special techniques, the mathematical burden is sufficiently light to be borne by all physics students, not only those theoretically inclined. The course need not be of excessive length if it is understood as exclusively an introduction to field concepts and hence not to cover in any detail the electric and magnetic property of materials. A number of particular ideas arising from the author's teaching experience are discussed.

This paper shows that the coherence function associated with a band-limited optical spectrum is expressible as an infinite product involving its zeros. Only a finite number of zeros are physically significant and these can be determined from measurements of the visibility of interference fringes; but an ambiguity remains in the sign of the imaginary part of each zero. If the spectrum is to be recovered using the visibility measurements, an auxiliary experiment is needed to supply the necessary signs, but it need not be especially accurate. If the signs are not known there is only a limited number of different spectra that are compatible with the visibility measurements. Finally it is suggested that wavelength measurements made on an asymmetric spectral line with a Michelson interferometer may yield differing results when used with long and short delays.

Segrè in his recent book [1] on Fermi gives an interesting account of the discovery of slow neutrons at the Institute of Physics of the University of Rome on October 22, 1934. He quotes (p. 80) a statement Fermi made many years later in a conversation with S. Chandrasekhar on the process of discovery in physics:

‘I will tell you how I came to make the discovery which I suppose is the most important one I have made. We were working very hard on the neutron-induced radioactivity and the results we were obtaining made no sense. One day, as I came to the laboratory, it occurred to me that I should examine the effect of placing a piece of lead before the incident neutrons. Instead of my usual custom, I took great pains to have the piece of lead precisely machined.

Norman Feather was one of the first physicists to realise the power of the coincidence method for elucidating nuclear level schemes. He observed coincidences occurring during the sensitive time of a Wilson cloud chamber in the 1920s. This note reports an application of some recent advances in coincidence technique in an attempt to set upper limits to the intensities of some unconfirmed γ-rays of 144Pr, which are indicated by broken lines in text-fig. 1.

The state of knowledge of fast neutron polarisation due to elastic scattering is reviewed in relation to the optical model of the process. There is clearly a substantial lack of polarisation data at neutron energies and concerning nuclei appropriate to the optical model description. The situation for the extremes of small and of large angle scattering is also considered.