Table of contents

The intensity distribution within the reflected fringe system from a multiple-beam interferometer employing silvered reflecting surfaces is discussed in its dependence upon the reflectivities and phase conditions at the reflecting surfaces. Using the Fizeau fringes of equal thickness localized on the zero order Feussner surface, the reflected intensity distribution is examined experimentally as the reflection coefficients of the interferometer surfaces vary between 4% and 90%. In the range of low reflectivities, when the silverings used showed marked colours, the changes in the reflected system lead to the measurement of a phase quantity related to the optical constants of the silver in the form of the thin film. Also in this range, the reflected fringes have a symmetrical form which is of use in the examination of sources of low intensity. In the range of high reflectivities, the conditions for the use of the reflected fringes in topographical investigation are discussed. The findings apply to fringes of equal chromatic order as well as to Fizeau fringes in reflection.

The change in optical transmission of a solution of colloidal graphite in a magnetic field has been measured as a function of a field strength and of concentration, and the results obtained compared with those to be expected from theory.

It is found that the experimental results agree closely with the hypothesis that the change in transmission is due to the change in projected area of the plate-like particles when these are aligned by the magnetic field.

By comparison of theoretical and experimental curves the particle volume is found. The rate of decay, after removal of the field, of the change in transmission produced by the field has also been measured.

An approximate theory is given to connect this rate with the diameter of the particles, and from this the size of the particles has been calculated. The dimensions so found agree satisfactorily with evidence from other sources.

A general, synthetic method of obtaining rigorous solutions of steady electron flow subject to space-charge forces is presented. The solutions are not obtained for given boundary conditions, but the boundary conditions are deduced from the solutions. Two examples of such solutions, involving strongly curved two-dimensional electron trajectories, are given; the method is in principle capable of giving the solutions of all possible electron flow patterns in three dimensions except perhaps those involving intercrossing trajectories. It is suggested that the subject offers scope for applied mathematical research at least on the same scale as potential theory.

A review of proposals made for altering the formula due to König, on which are based measurements of air-particle velocity by Rayleigh disc, leads to the conclusion that there is insufficient evidence for altering the numerical constant in the formula. Comparisons between Rayleigh disc and other methods of calibration of individual microphones are discussed, and the possibility of difference due to reaction on the sound pressure by the vibration of the diaphragm is considered.

A new type of gravimeter is described, which depends for its operation on the change of frequency of the natural vibration of a vertical wire stretched by a weight. The advantages of such an instrument are presented, and a description given of the design which has so far been evolved. The results of laboratory tests and of trials carried out at sea in H.M. Submarine Talent are given.

In order to determine whether any structural change takes place when a liquid is supercooled, measurements have been made on the viscosity of pure phenyl ether, which by its melting point (26.85° c.) is suitable for accurate work. It has been found that in both the ordinary and the supercooled state the liquid obeys a formula of the type η=A exp (b/T), but that the value of b in the supercooled state, which was investigated down to 17° c. below the melting point, is significantly larger than the value of b for liquid above the melting point. It is shown that an accurate value for the melting point can be obtained from viscosity measurements without the liquid ever becoming solid. The temperature coefficient of density shows no detectable change in passing through the melting point.