It is difficult for a material having low resistance to electron beam irradiation to obtain an electron microscopic image having a high SN ratio. A screen protector is mounted over the screen. In the preferred embodiment, the aquatic ladder includes upper railings which each have an inverted \u201cU\u201d shape, and the distal end of those railings have hinged brackets for mounting the aquatic ladder to an object to be boarded. First water softener 4 is regenerated by salt water from a salt water chamber 14. The preceding penalties of all preceding lines are defined by a previously calculated optimal line break scheme.