A device, to be attached to a telescope or microscope, consisting of a hairlike filament (hair, segment of spider-web or wire) connected to a screw so that as the screw is turned, the filament is translated across the field of view and in the focal plane of the telescope or microscope. The screw has a finely-cut thread and the amount of rotation is indicated by graduations on the head of the screw or by other means. The screws used in micrometers for geodetic instruments are graduated in angular measure (seconds, on American instruments). Turning the screw moves the filament a distance corresponding to the angle, at the instrument, between the two positions of the filament. Any small angle can be measured by noting the whole and fractional number of turns of the screw made in moving the filament from one position to the other and multiplying this by the angular value of one turn.