A display apparatus comprises a shift register array. The system also permits user to interrelate information items from different locations or different tools. The housing additionally includes a reference chamber holding a gas a second pressure lower than the first pressure. In this respect, the light beam projector is mounted on the holder for projecting a beam of light at a right angle relative to a lengthwise axis of the holder such that the beam of light is parallel to the straight outer edge of the guide bar. There is at least one event causing each state transition, and the actions which occur upon a state transition are dependent upon the event that caused the transition.