The present invention relates to improved means for measuring and recording small angular motions of an object, such as the muzzle end of a long-barrel artillery piece or gun during firing.  One of the problems associated with such measurement in the case of gun barrels is that of preventing the recoil movement of the gun barrel from introducing an error into the measurement.  Another problem is to resolve the angular motion into two components, such as horizontal and vertical, and separately measure these two components accurately. 
In accordance with the present invention: a first mirror is rigidly mounted on the gun muzzle, or other movable object, in a predetermined plane; a collimated primary light ray is projected onto the mirror in a direction nearly perpendicular to the mirror plane, whereby the ray is reflected back along a path close to the primary ray, with horizontal and vertical components of motion produced by the angular motion of the object; the reflected ray is split by optical means into two spaced substantially-parallel component rays having transverse components of motion in two spaced substantially-parallel planes, for example, vertical planes; the two component rays are reflected by another mirror in directions transverse to the two parallel planes, for example, horizontally; and the transverse components of motion are measured by a photosensitive sensor in the path of each reflected component ray. 
The reflected primary ray is preferably reflected by about 90.degree.  by a third mirror positioned near the source of the primary ray.  The ray-splitting optical means may be a group of three mirrors, one of which is half-reflecting and positioned at a 45.degree.  angle to the reflected primary ray to permit part of the ray to pass through as a first component ray and to reflect part of the ray at 90.degree.  thereto as a second component ray, the second is positioned to reflect the second component ray in a direction normal to the plane of transverse motion of the reflected primary ray, and the third is positioned to again reflect the second component ray in a direction parallel to the reflected primary ray. 
Preferably, the mirror that reflects the component rays to the photosensitive sensors is a plural-sided mirror which rotates about an axis parallel to the direction of transverse motion of the two component rays at a relatively high speed, and each sensor comprises a V-shaped photosensitive element which is scanned by one of the reflected component rays.