The package contains a series of cards in pairs. The pair of cards has the same picture, except that on one card an item has been added or subtracted from the picture. In the example here, on one card, there is ++ and on the other... Using stereoscopic glasses, the child must look at the two pictures and try to connect them with his eyes until they overlap so that he sees one picture in which he also sees the additional item. When this happens, the examiner, using the measuring tape below the pictures, finds the distance between the pupils and decides on further treatment. The method was developed by Prof. Dr. C.I. Sattler in 1928 in Germany