" Dioptra ," says Venturi , were instruments which had great resemblance to our modern theodolites . The book Dioptra is a treatise on geodesy containing solutions, with aid of these instruments, of a large number of questions in geometry , such as to find the distance between two points, of which one only is accessible, or between two points, which are visible but both inaccessible; from a given point to draw a perpendicular to a line which cannot be approached; to find the difference of level between two points; to measure the area of a field without entering it.