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A mass spectrometer of the Nier type has been designed and constructed for the identification of isotopes and measurement of their relative abundances. This instrument employs a universal gas-type ion source which may readily be replaced with a furnace or filament-type ion source for use with solids. The instrument is compact and portable with the exception of service connexions and a galvanometer shelf. The electronic units designed for this instrument are described in detail. These units operate satisfactorily from unstabilized a.c. and do not require batteries.

The mass spectrometer is capable of detecting isotopic components present to the extent of 1 part in 100,000 of a sample. With the present slit dimensions, the instrument was found to have a resolving power better than 1 in 130, which agrees with that calculated from the slit dimensions and the radius of curvature.

An instrument has been developed for the testing of camera shutters of either `between-lens' or `focal-plane' types, in which a curve is drawn on the screen of a cathode-ray tube; vertical deflexion of the trace corresponds to increase of light passing through the shutter, while horizontal deflexion is proportional to time. No synchronization is needed between the shutter and the instrument, the time base starting immediately the shutter blades begin to open. After the curve has been drawn, the time base makes a second traverse, closing in the base-line of the pattern and adding vertical timing marks at 100, 10 or 1 msec. intervals. The accuracy of these timing marks is maintained by the a.c. mains supply. The screen of the tube has an after-glow property, and the curve remains visible for several seconds. Owing to the very great range of `writing speeds' of the spot on the screen when testing different shutters, a circuit has been incorporated which increases the intensity of the electron beam in the tube in proportion to the speed of deflexion of the spot and thereby maintains an even brightness at all parts of the trace. A miniature discharge flash tube has been built into the instrument and may be operated at any point in the traverse of the time base. This is of value for inspecting the shutter and its mechanism at any point in the operating cycle. Provision has been made for using the instrument for checking the synchronization of shutters to be used with flash bulbs.

This paper describes an improved model of the instrument of which details were given in an earlier number of this journal. The function of the instrument is to give permanent records on photo-sensitive film of the development of pressure caused by the explosion of gaseous mixtures. The maximum deflexion available is about 8 cm. and can be made to correspond to a wide variation of pressures. Pressures developing at the rate of 50 lb./sq. in./msec. are readily recorded. The natural frequency of the recording system (in which there are no loose-coupled parts) is high and is an inverse function of the sensitivity.

The possibilities of the absolute determination, by radioactive track-count methods using `loaded' photographic plates, of decay constants, nuclear reaction cross-sections, nuclear particle fluxes and concentrations of radioactive bodies are discussed. Experiments are described showing the influence of conditions of loading on the amount of material absorbed by the emulsion, taking uranium as a representative loading material. It is found that the velocity of the uptake decreases rapidly with time of immersion. The quantity absorbed is a nearly linear function of the uranium concentration in the bath over a concentration range 3.3-0.01%, but at lower concentrations absorption is relatively more efficient. In neutral solutions absorption is larger than in slightly acid solutions, possibly owing to hydrolysis. High concentrations (similar 20-50%) of a neutral salt (ammonium formate) depress the uptake, probably owing to de-swelling of the gelatin. Lead ion in up to 80-fold excess does not displace uranyl ion in the emulsion.