Abstract: Displays of first and second points are generated on a fluorescent screen. A first mark, which coincides with the display of said first point, is displayed on said fluorescent screen. A linear second mark is displayed on said fluorescent screen. A parameter which controls the position of said second mark on said fluorescent screen is varied in a known manner to move said second mark to a plurality of positions, in each of which said second mark constitutes a display of a circle which is centered on said first point, inclusive of one position in which said second mark is in close proximity to the display of said second point, said parameter having a predetermined, known relation to the radius of said circle so that the value of said parameter is a measure of the distance between said first and second points when said second mark is in said one position and said first mark coincides with the display of said first point.