Glass sheet, accurately curved in two directions and having a compound curved surface, provides a non-cylindrical, non-spherical, smooth, continuous reflection image. Multivalued memory operations can be conducted at a high speed and with stability by applying a certain positive voltage to a gate electrode to accumulate electrons in the silicide quantum dots and by applying a certain negative voltage and weak light to the gate electrode to emit the electrons from the silicide quantum dots. A stainless steel slip joint member is attached to an end of the low-carbon steel cylinder to facilitate joining adjacent pipe sections.