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An electron gun is described capable of producing a ribbon-shaped beam of electrons moving under crossed electric and magnetic fields.

The characteristics of the gun are derived by applying the adiabatic principle to motion in constant uniform crossed fields.

The paper gives an account of measurements made on the lattice parameter of cobalt when the material contained crystal grains of different sizes as found in fine dust, filings and annealed solid rod. The stable structure of cobalt was found to depend upon the grain size: between room temperature and about 450°c when the grain size is very small, as in cobalt sponge, the stable structure is face-centred cubic; when the grain size is larger, as in a solid rod, the stable structure is close-packed hexagonal; when there is a range of grain size, as in fine filings a mixture of these two structures is observed. At temperatures above 450°c cobalt sponge is still face-centred cubic, and the filings and rod also now show face-centred cubic structure with the same lattice parameter (3.5370kx at 18°c). The lattice constants of the hexagonal close-packed structure at 18°c are: a = 2.5003kx, c/a = 1.632₂.

Different results were obtained according as the material was maintained and examined at elevated temperature or was quenched from elevated temperature and examined at room temperature. All specimens of cobalt, whatever the grain size, annealed at about 1000°c and quenched showed a mixture of the two structures although with cobalt sponge the amount of the hexagonal structure was very small compared with the amount of the cubic structure.

Combining these results with recent results by Newkirk and Geisler (1953) it is possible to present a fairly complete picture of the effects observed, between room temperature and 1220°c, with material examined at elevated temperature and with quenched material examined at room temperature. The main results fit the theory of the imperfect hexagonal structure put forward by Edwards and Lipson (1943).

A cubic face-centred structure with lattice parameter (3.5540kx) greater than that of the cubic lattice ordinarily found (3.5370kx) was observed with filings quenched from temperatures between about 600°c and 840°c. It is suggested that the material giving this lattice parameter is in a metastable state.

The thermal expansion of cobalt sponge is represented by a smooth curve the equation of which, for temperatures up to 650°c, is the quadratic a_t = α₀(1 + αt + βt²), where a₀ = 3.5362 kx; α = 12.297 × 10^-6; β = 2.042 × 10^-9. It shows no transformation such as is found at about 450°c with material in solid form. No other high-temperature transformation was observed.

The surface distortions accompanying the familiar ring crack Hertzian cones formed on glass by local pressure are studied optically for four different optical glasses. The ring cracks are produced both with a rounded diamond and a steel ball indentor. The surface profiles are studied both by interferometry and with the light profile microscope. A frequent observation is the occurrence of multiple circular ring cracks surrounding a flat, slightly depressed disc, as a series of annular pile-ups of glass, usually not exceeding a height of 1500 Å. Surface profiles are illustrated for individual cracks on each of the glasses. The area of contact between glass and steel at the onset of a ring crack on a typical glass is revealed by an interferometric method. The crack appears to start at the rim of the circle of contact and sets in at a mean pressure of 14000 kg cm^-2.

Two octahedral faces of a large very perfect diamond have been studied optically and interferometrically. On the surfaces have been found a hundred crater-like features each about 50 μ wide and a few hundred ångstrom units high. It is shown that these are the analogue on diamond of the long familiar ring crack Hertzian cones formed on glass subject to local pressure. It is possible that impact with other diamonds is responsible for the ring crack formation. The cracks on the diamond are crystallographically oriented and the profiles of some are measured interferometrically.

The influence of variations in colour vision on matching a dyed pattern with one of a range of dichroic series has been observed for 250 normal observers. The results have been compared with the variations produced by a change in lighting for a single observer. The results have been related to the data of Wright on the variations in colour vision attributable to differences in pigmentation with which they are in close accord. The number of observers was sufficient for a statistically significant change in colour vision with age to be detected. This change corresponds to increased pigmentation and confirms the commonly held view that older people see redder than young. The bearing of the results on the selection of colour workers is discussed.

Experiments have shown that when the current through a positive column in mercury vapour falls below a certain limit, the longitudinal field does not remain constant, but increases with decreasing current, at first slowly and later more rapidly. This is due principally to failure of the discharge to establish a plasma with approximately equal concentrations of ions and electrons. The properties of the positive column have been formulated without making any initial assumption about the concentrations. A complicated system of differential equations with boundary conditions results, which determines the concentration of the charged particles, and hence all the other properties of the discharge. Schottky's theory of the quasineutral column appears as a limiting case. To obtain a preliminary solution of the equations, which need normally mechanical integration, the assumption has been made that, to a first approximation, the concentrations are in a constant ratio. The equations can then be solved, and in particular expressions obtained for the current versus longitudinal field characteristic curve for the column, and for the onset conditions for the special properties of the low-current form of the column. Fairly good agreement has been found between the numerical predictions of the theory and the results of experiment. It has also been found that with decreasing current density the field approaches a limiting value which is obtained from the usual Schottky formula simply by replacing the ambipolar diffusion coefficient by the electron diffusion coefficient.