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Hitherto, no palaeomagnetic data for India are available earlier than the Jurassic, 130 M.y. The remanent magnetic directions of oriented samples from one Jurassic and four pre-Cambrian rock formations in India have now been determined. One pre-Cambrian formation gave very scattered direction : the other four formations gave well-defined directions and their magnetic stability was demonstrated by a.c. and thermal demagnetization. The three pre-Cambrian results enable values of the ancient latitude and orientation of India to be estimated at 500, 600 and 750 M.y., assuming the ancient field to have been an axial dipole.By making use of these new results, together with similar data by other workers for Australia, North America and Europe, it can be shown that, as far as the scanty data goes, the rate of drift in latitudes and orientation of the four continents has been of roughly the same magnitude throughout the period since 750 M.y.

Photomicrography may be readily accomplished by standard techniques. Exposure times for luminescent photography, using High Speed Ektachrome film, range from one-half second to about one minute. A cadmium sulphide cell ohm-meter combination has been used for estimating exposure times. Some light originating in the air discharge does reach the specimen; and for critical work and the best quality photography, this stray light can be largely removed by use of a gas other than air to support the discharge. This other gas may be admitted through the mechanical leak. Both hydrogen and helium are superior to air in this respect, but hydrogen works best. Using a hydrogen discharge at the optimum pressure, only a very dull red discharge can be observed in the lower region of the discharge tube. This allows only minimal light to reach the specimen. Even this minimal amount could effectively THE REVIEW OF SCIENTIFIC INSTRUMENTS be removed by incorporating an interference filter in the optical path.Although the instrument has been developed primarily for observation of geological specimens, it should prove useful for luminescent observations of other substances. Applications are likely to be found in such fields as ceramics, cements, synthetic crystal production, and particularly in the area of solid state physics research. ACKNOWLEDGMENTSThe author is indebted to Professor J. V. Smith who discussed freely the properties which a luminescence device should have, and in particular, very graciously relayed to the author the experience of Dr. J. V. P. Long who found that a simple, poor vacuum, alternating current device was feasible.A hysteresis loop tracer employing an alternating current electromagnet capable of providing fields up to 3600 Oe is described. Merits of the present system over other types of loop tracers are pointed out. Some typical hysteresis loops obtained using the apparatus are also presented.

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A simple method for an in situ measurement of susceptibility and low-field hysteresis on rock bodies like dykes and flows is described. Extensive use of this method indicates that in some dykes and flows, the nature of the magnetic grains varies rapidly across the bodies. Making use of such observations, it seems possible to select stable rock samples for palaeomagnetic work and also understand to some extent the origin of remanent magnetization in different types of rock bodies.

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