Table of contents

The influence of crystal perfection on extinction is briefly discussed and the possible corrections given. Experimental results for chemically produced copper and annealed aluminium filings have been obtained with a Geiger counter spectrometer using monochromatic radiation. The intensity measurements, and for copper the experimentally determined particle size, were inconsistent with the views that primary extinction occurred, but could be interpreted wholly in terms of secondary extinction, the corrected f values agreeing within ±2% with accepted values. Evidence to show that annealed metals contain residual lattice strain is advanced, in accord with the dislocation theory, and it is suggested that secondary extinction will predominate over primary extinction in all crystals containing such dislocations.

Measurements have been made, using a Geiger counter spectrometer, of the changes produced by cold work on the integrated intensities in the Debye-Scherrer spectrum of aluminium. It is shown that cold work reduces the integrated intensity of all reflections, this decrease being exactly compensated by a rise in the diffuse background scatter, so that the total reflected intensity is unchanged. These results are at variance with those of previous workers, but the discrepancy can be readily explained by the absence of a correction for extinction in previous work. Brief reference is made to measurements of line width and it is shown that the results support the dislocation theory of plastic deformation.

In order to resolve some inconsistencies in the literature on the second Brillouin zones of cubic lattices, the shapes of these zones have been recalculated. The results, together with descriptions of the third and fourth zones, are given

The paraxial lens data and the more important first order lens aberrations were determined for a series of symmetrical magnetic lenses of equal lens diameter D but variable air gap width S for a wide range of lens excitations. The range of gap widths covered was S/D=0.2 to S/D=2 The field distribution within the lenses was measured to a high degree of accuracy with a resistance network analogue, and the lens data were then obtained through numerical trajectory tracing. The results are represented as a series of graphs These results are representative of the most common magnetic electron lenses, and should have useful applications in lens design. An approximate formula for the focal length is fapproximately equal to50(DS)^1/2V_r/(NI)². A few design examples are discussed.

Using results recently published it is shown that each of the investigated symmetrical magnetic lenses has an optimum value of the lens excitation parameter, for which the spherical aberration constant is smallest. Taking account of the dependence of the maximum obtainable axial field strength H_o on the maximum field strength H_p in the pole piece gap, one finds that there is an optimum design of electron microscope objective lens of conventional type which gives a lowest value of spherical aberration and highest resolving power. A modification of this optimum lens as adapted for practical use is described.

The variation of photographic blackening with intensity, exposure, the thickness of material round the film, the angle of incidence and the wavelength of X-rays, are examined theoretically and, with one exception, experimentally A detailed theoretical investigation is made of the energy absorption in the silver bromide grains, consideration being given to the energy lost by scatter and fluorescence and to the electron interchange between the silver bromide grains and the surrounding gelatine and other material. It is concluded that for the majority of emulsions there will be no variation of blackening with X-ray intensity and that the photographic density will be proportional to the exposure at low densities. At very short wavelengths the blackening is critically dependent upon the thickness of material round the emulsion. Photographic density decreases with increase of angle of incidence of X-rays, the maximum decrease observed being about 17% Within the wavelength range examined experimentally (0.020 to 0.354 A.) the photographic density was approximately proportional to the energy absorbed by the silver bromide. The density per röntgen varied by a factor of 42 in this range. The variation of density per röntgen with wavelength and angle of incidence limits the applications of photographic methods of X-ray measurement. Methods by which emulsion manufacturers can reduce these variations are suggested.

Previously reported results of polishing and chemically etching the surfaces of germanium crystals are confirmed The effects of vacuum heat treatment are investigated and it is found that treatment of this kind at temperatures between 500 and 900°C. can improve the rectification characteristics considerably. The treatment does not spoil the optical quality of a polished surface and in this connection may provide an advantage over chemical etching. Similar effects are obtained with deficit-conducting lead sulphide Modifications to the surface structures of the crystals by various treatments are investigated by electron diffraction

A method is described for obtaining dynamic values of Young's modulus at low frequencies (20 c/s to 100 c/s) by using the forced resonance of a vibrating cantilever. Provision has been made for keeping the apparatus at a constant temperature and a few measurements of the dynamic modulus are given for polythene at 30°C and 40°C The theory of the vibrations of a cantilever including viscous internal damping is discussed. The results continue previous work carried out at higher frequencies by a different method and indicate the continuity of the dynamic modulus-frequency relation between 20 kc/s and 20 c/s for polythene `Static' measurements have also been carried out and agree with those published earlier Even at the lowest frequency the dynamic modulus is still three times the `static' value, and it is concluded that lower frequencies should be investigated

A simple new design of interferometer based on the wave-front shearing principle is described and illustrations given of its application to the solution of a variety of `through-type' and autostigmatic testing problems.