Compact optical sighting level with internal fine setting of the horizontality of the line of sight

Claim 1

An optical sighting level comprising an objective lens system of which the focal distance is F, a mirror disposed between the lens system and its image focus point at a distance F/2 from said focus point and adapted to reflect in an upward direction the light beam having passed through said objective lens system, a vertically movable divergent lens disposed above said mirror with its optical axis perpendicular to the optical axis of said lens system at its intersection point with the reflecting surface of said mirror, the plane containing said two optical axes being vertical when the level is set in its sighting position, a prism reflecting in said vertical plane the optical axis of said movable divergent lens, firstly upwards and forwards by an angle of 60*, and secondly upwards and backwards by an angle of 120*, said second reflection being produced by a roof-shaped second reflecting surface of said prism to reverse the observed image from side to side, an eyepiece of which the optical axis is coincident with said twice bent axis of said movable divergent lens, a cross-wire located at the focus point of said eyepiece, and means to adjust the position of said mirror around an axis perpendicular to said objective axis and to said movable lens axis at their intersection point on the reflection surface of said mirror, so as to keep the sighting in a true horizontal direction.