Abstract: Instrument for measuring the distances of points from a plane or a straight line defined by means of the instrument. The instrument may also be used in marking output points from a plane or a straight line.The instrument functions by forming at least two mirror images of a measuring object in such a manner that these images are differently oriented when viewed. This is accomplished in the simplest embodiment of the instrument by a prism whose front face is semi-reflecting whereas the other faces are perpendicular to each other and totally reflecting. If the measuring object is not on the defined plane or line this will show up as a displacement of the differently oriented mirror images in relation to each other.The substantial novelty of the instrument is that all the images the relative displacement of which are employed for the read-out are mirror images of the measuring object and are formed without the need for focussing optical components such as lenses and concave mirrors.