In order to realize an optical attenuator which has an attenuation factor independent of the wavelength of incident light, a glass substrate is used in an attenuator plate, and the glass substrate is formed in a tapered section so that its incident and exit planes may not be parallel to each other. The screen can be formed in a top face of a computer and a keypad can be a split on opposite sides of the screen. The contracting parties act complementary, money and goods or money and services are never at the same time under control of one contracting party.