It has long been known that positive ions are capable of ejecting electrons from a metal surface, and it has recently been shown by Oliphant that metastable atoms of helium are capable of producing the same effects. In view of the importance of these phenomena in the theory of discharge processes and of their intrinsic interest, it is desirable to consider the theory of the effect in the light of the Sommerfeld theory of metals and the analytic methods of wave mechanics.

We are concerned in this note with a problem which may be regarded as a generalisation of one which occurs in the theory of Fourier series.

The Heine-Borel Theorem for one-dimensional intervals may be enunciated as follows:

(A) If a set D of intervals, all in the closed interval (a, b), be such that every point of (a, b) is an interior point of at least one interval of the set D (the end points a, b being regarded as interior to an interval when either of them is an end point of such interval), then a finite set E of intervals all belonging to D exists such that every point of (a, b) is interior to at least one interval of E.

Let c0, c1,…, cn,…be a sequence of real constants, and ƒ0(x), ƒ1(x),…, ƒn(x),… a sequence of functions defined, for example, in the interval (0, 1). In this paper we shall investigate some of the properties of the series

which may be obtained from the standard series

by interchanging the signs of the terms in a quite arbitrary way.

For dealing with atoms involving many electrons the accurate quantum theory, involving a solution of the wave equation in many-dimensional space, is far too complicated to be practicable. One must therefore resort to approximate methods. The best of these is Hartree's method of the self-consistent field. Even this, however, is hardly practicable when one has to deal with very many electrons, so that one then requires a still simpler and rougher method. Such a method is provided by Thomas' atomic model, in which the electrons are regarded as forming a perfect gas satisfying the Fermi statistics and occupying the region of phase space of lowest energy. This region of phase space is assumed to be saturated, with two electrons with opposite spins in each volume (2πh)3, and the remainder is assumed to be empty. Although this model hitherto has not been justified theoretically, it seems to be a plausible approximation for the interior of a heavy atom and one may expect it to give with some accuracy the distribution of electric charge there.

Formulae for multiple tangents to the general surface in ordinary space were obtained at different times by various writers, but the discussion remained incomplete until the advent of Schubert's enumerative method, which solves the whole problem by a purely mechanical process. Schubert later extended the method to general forms in [n], in the restricted case where the multiple tangents have only a single contact. In the present paper the results for three dimensions have been used to build up the formulae for forms in [4], by means of the correspondence theorem on which much of Schubert's work is based. It would no doubt be possible to evolve a complete set of incidence formulae for four dimensions and then to proceed as in Schubert's discussion of surfaces; but the present method is preferable for two reasons. In the first place, all the results have been obtained in a very simple manner; and secondly, a large number of minor results have been found in the process.

According to the views which we expressed recently (Proc. Roy. Soc. 1929, 124 A, 322) the solid condensed, liquid condensed, expanded and vaporous types of unimolecular films on the surface of aqueous solutions are the two-dimensional analogues of the solid, liquid crystal or smectic, liquid and vapour states of bulk matter. The previously-unsuspected observation that the films show triple point phenomena supports these conclusions.

The principal object of the present paper is to give a geometrical account of a certain Cremona transformation of four-dimensional space ([4]) into itself. The transformation in question is analogous to the cubo-cubic transformation of [3] which is obtained by drawing cubic surfaces through a twisted sextic curve of genus three, . A similar transformation, in space of any number of dimensions, has been described analytically in a note by Godeaux; and most of our work in the present paper extends to the general case. The case of four dimensions is studied here as being less abstract, and more easily susceptible of geometrical treatment; in the general case it is less simple to dispense completely with algebraical methods. We shall see that the various loci which arise in the course of the work are already well known.

It has been remarked in a former paper that perhaps a fuller understanding of the theory of linear groups may be arrived at by considering the real representations. It is sufficient that these be irreducible in the real field. In the present paper we continue this investigation and deal with the angles of rotation; in particular we find the form of the commutator

where S and T are substitutions of order p1 and p2 and have angles of rotation θ, θ′, θ″, … and φ, φ′, φ″, …. If certain conclusions regarding the orders of S, T, and C may be drawn we shall then be able to attack a very important problem—;that of Primitivity. This is in essence very similar to the method of Blichfeldt, since the commutator is the product of

which are both of order p2.

We consider in space [3] a curve C of order n and genus p without multiple points. If we represent the lines of [3] by the points of a quadric Ω in [5], the chords of C will be represented by the points of a surface F of order (n−1)2−p lying on Ω. This surface has a triple curve M (with multiple points) corresponding to the ruled surface of trisecants of C (and the quadrisecants) of order ⅓(n−1)(n−2)(n−3)−p(n−2). It is the object of this note to find the genera of M and of a prime section ϑ of F; these being also the genera of the ruled surface of trisecants of C and of the ruled surface of chords of C which belong to a linear complex.

It is well known that the rotational specific heat of a diatomic gas is given by

where R is the gas constant, σ = h2/8π2AKT, h is Planck's constant, T is the absolute temperature, K is Boltzmann's constant, and A is the moment of inertia of the molecule.

For r>-1, let denote

If

we say that the series a0 + a1 + a2 +…+an+… is summable by Cesàro mean of order r, or more shortly summable (C, r) to sum s. If r >−1, and

we say that the series is summable by Rieszian mean of order r to the sum s. It has been shown that these two methods of summation are equivalent. Throughout this paper I shall deal with the Rieszian mean, but I shall retain the symbol (C, r). It is known† that if a series is summable (C, r), it is also summable (C, r′) to the same sum for all numbers r′ greater than r.

The Fourier series of the function defined by

is

and if

then

while

The expression (1) exceeds (2); this is the Gibbs phenomenon.

The intensity of stifling is estimated. Under ideal conditions the absorption should vary as the secant of the angle of incidence of the sound.

When a vessel of liquid has been emptied and put aside, a thin film of liquid clings to the inside and gradually drains down to the bottom under the action of gravity. The layer being thin, the motion is very nearly laminar flow, and the curvature of the surface in a horizontal direction may be ignored. Thus the problem for a cylindrical vessel is reducible to that of a wet plate standing vertically.

Continued fractions were generalised to more than one dimension by Jacobi and others: later Perron gave an account of the existing state of the subject with a detailed discussion of periodic fractions. Quite recently the subject has been attacked afresh by Mr. Maunsell

Many measurements have been made of the complete absorption curves for β-rays in various substances, and it has been found that, for homogeneous β-rays, the curve is not exponential, and, in particular, when aluminium is the absorber, the curve is linear over a considerable portion of its range (e.g. Schonland, etc.).

It is generally believed that isolation has played an important part in evolution. If an organism is to evolve so as to adapt itself to a special type of environment, e.g. a cave or a desert, it must not be swamped in each generation by migrants from the original habitat.