When this instrument is in its observing position the visible horizon is reflected at three points vertically downwards into a telescope. The portions of the horizon observed are facing the observer and lie 60° to his right and left. The images of the visible horizon appear as three straight lines in the field of view intersecting at about 60°. The observation consists in bringing the image of the celestial object into a certain point defined by the configuration of the horizon-lines. It is of no consequence whether all the rays after the downward reflection are again reflected into a horizontal direction, and whether this occurs before or after the rays enter the object-glass. It will be shown below how the observer is enabled to keep the error of inclination so small that its effect on the result, of second order, can be neglected.