The Secretary of State for Industry in Her Britannic Majesty's

James Morriss Burch

David Charles Williams

The invention comprises an afocal lens system having unitary magnification hich is either positive or negative and suitable for indicating departures of a reference point or a reference axis respectively from coincidence with a relatively fixed datum axis. The reference point or axis is included by the axis of the lens system, and the datum axis is defined by a line through an object and the zero position of an image of that object produced by the lens system. The lens system indicates the said departures from coincidence by means of shifts of the image from the zero position. The positive magnification afocal lens system is sensitive to tilt but insensitive to displacement, whereas the reverse is true of the negative magnification version. This feature allows tilt and displacement to be monitored separately.

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James Morriss Burch