SummaryThin crystals of amethyst and citrine were examined by transmission elflf'troJl microscopy. It was found that all crystals became damaged during observflti"ll. The diffraction contrast from the damage centres indicated that these were RmfLll amorphous regions. Electron spin resonance observations of these crystals show that the amorphous regions are associated with Fe3+ ions; the damage centres provide, therefore, a measure of the distribution of this impurity in the crystals. Brazil twin boundaries, generally of the order of 1000 A apart, were observed directly. Radiation damage occurred preferentially at the twin boundaries suggesting that there was a concentration of impurities along them. The relevance of these observations to the optical properties of the crystals is discussed briefly.

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SummaryIn an attempt to correlate the degree of plastic deformation of chromium with the physical properties associated with its antiferromagnetism, a study has been made by thin-foil electron microscopy of the dislocation substructures in polycrystalline chromium sheet after deformation by rolling and subsequent annealing. The original hot-rolled sheet exhibited a typical cell structure. Annealing at 900°C produced regular hexagonal networks and simple tilt boundaries. Complete recrystallization was achieved by annealing at 1250°C. 1· 5% defor=ation of this material at room temperature produced a non-uniform substructure of sharply kinked dislocation lines and irregularly shaped closed loops. Annealing at temperatures below 650°C had little effect. However, at that temperature there was some recovery and large circular loops lying in the slip planes appeared. The beginnings of regular networks appeared and the closed loops annealed out at 900°C. Comparison with the annealing characteristics of dislocation loops in iron allowed an estimate of the activation energy for the process in chromium to be made. It is shown that the elastic stress induced by plastic deformation, which critically affects the physical properties associated with the antiferromagnetism, is consistent with the experimental observations of the influence of an applied magnetic field on the elastic properties of chromium. I. INTRODUOTIONPrevious work (Bacon 1961;Street 1963) has shown that the physical properties of chromium associated with its antiferromagnetism are strongly dependent upon its state of plastic deformation. In an attempt to correlate the degree of plastic deformation with the observed antiferromagnetic properties, the dislocation substructures in annealed and plastically deformed chromium have been studied by thin-foil electron microscopy. II. EXPERIMENTAL PROOEDUREThe specimens were made from 99·97% pure chromium supplied in the form of hot-rolled sheet 0 ·002 in. thick. All annealing treatments were carried out in a vacuum of approximately 10-5 mmHg. The specimens were furnace cooled.Thin foils suitable for transmission electron microscopy were prepared by electropolishing in a solution of 65 ml perchloric acid (sp. gr. 1·54), 300 ml acetic acid, and 300 ml butyl cellosolve at about 1°C, in an apparatus similar to that described by Presland (1961). The cathodes were stainless steel and a current of 1·5 A from a current-stabilized rectified a.c. power supply was used. The thinned foils were examined in a JEM-6A electron microscope operating at 100 kV.

antiserum, one of which displays activity against a variety of non-neural human tissues, while another seems to be specifically antiglial. In these studies, which are still in progress, the response of neurons to the antiglial antiserums has not yet been evaluated.

Measurements have been made of the spectral distribution of the intensity of light reflected from a blue and a red labradorite for several angles of incidence over the wavelength range 3000 to 7000 A. Transmission electron microscope observations of thin crystals of these labradorites show that the crystals consist of alternate light and dark lamellae of the order of 1000 thick. Although the pattern of the lamellae appears almost periodic, a statistical analysis indicates that the structure has no long-range order. A theoretical discussion of the diffraction of light in such a structure is given and an expression for the spectral distribution of the reflected intensity derived. The spectral distributions to be expected from the blue and red labradorites have been computed using values of the mean thickness and variance of the alternate lamellae obtained from the electron micrographs. Satisfactory agreement between the observed and calculated spectral distributions is obtained. This supports an earlier suggestion that the colour of labradorite is due to optical interference in a lamellar structure. Die spektrale Verteilung der von einem blauen und roten Labradorit reflektierten Lichtintensitat wird fiir verschiedene Einfallswinkel irn Wellenliingenbereich 3000 bis 7000 a gemessen. Elektronenmikroskopische Beobachtungen an dunnen Kristallen dieser Labradorite zeigen, da13 die Kristalle aus hellen und dunklen, ungefiihr 1000 A dicken Lamellen bestehen. Obwohl das Muster der Lamellen meistens periodisch ist, zeigt eine stetistische Analyse, daB die Stniktur keine Fernordnung besitzt. Es wird eine theoretische Behandlung der Lichtbeugung in einer derartigen Struktur gegeben und ein Ausdruck fur die spektrale Verteilung der reflektierten Intensitat abgeleitet. Die zu envartenden spektralen Verteilungen fur blauen und roten Labradorit werden mit Werten der mittleren Dicke und Abweichung der Lamellen aus den Elektronenmikrogrammen berechnet, und befriedigende Ubereinstimmung zwischen den beobachteten und berechneten spektralen Verteilungen gefunden. Dies unterstutzt eine friihere Anregung, da13 die Farbe des Labradorits durch optische Interferenz in der Lamellenstruktur herruhrt.