Abstract: To measure distances, light from a flash tube is plane polarized by a polarizing filter, then polarization modulated by an electro-optical crystal, then passed along a `wobbling` light path, then subjected to a relative retardation of about a quarter wavelength by a rhomb and then passed through a calibrated variable light path before being projected to a distant target reflector. The reflected light returns along the same path and is detected by a photomultiplier after passing through the polarizing filter. Adjustment of the variable light path until a null is obtained at the photomultiplier indicates that the length travelled by the light from the crystal to the reflector and back is an integral number of modulation wavelengths, possibly plus one half-wavelength. The quality of the null is adjustable by adjusting the orientation of the rhomb. Temperature correction may be provided by constructing a reference resonator defining the modulation wavelength of the same material as a structure being measured.

Abstract: A terrestrial refractometer comprising a gastight enclosure of substantially invariant transverse section and having a linearly movable endwall which varies in position in accordance with ambient temperature and pressure, the enclosure containing a fixed mass of a gas which at least approximates to an ideal gas; sensing means responsive to the linear position of the endwall; and compensating means arranged to apply a compensating factor which is proportional to the linear movement by the endwall from its position under conditions of standard temperature and pressure.

Abstract: A terrestrial refractometer comprises a gastight enclosure containing a fixed mass of a gas which at least approximates to an ideal gas, at least a part of the enclosure being movable in response to changes in the volume of the gas in accordance with changes in ambient temperature and pressure; constraining means for constraining said movable part so as to maintain the gas at a constant volume; and sensing means for providing a signal related to the force applied to the movable part by the constraining means.