ABSTRACT: The author states the difficulties connected with the construction of very large reflectors and describes a mirror, composed by nineteen hexagonal elements. This instrument has been built and tested by the author at the Bologna University Observatory, with the mirror fixed in zenithal direction. Here the method followed in the adjustment of the elements is explained together with the description of the movements of the photographic plate. With this instrument photographs of the sky had been taken since 1935. HISTORY - The difficulty of building large mirrors essentially consists in the fact that the thickness must be proportionate to their diameter 1 because if the disc is too thin, the glass flexes and does not retain the shape given to it by the smoothing: this leads to masses very heavy, which are handled badly during processing, and when this is completed and the mirror is placed in its final bed, supporting it on the back with equal pressure at every point is a ver...