Abstract: A two piece plastic lens for a large screen projection system in which the two lens elements are formed with similar curves. The lens is supported at its peripheral edge by a flexible strip having a T-cross section that fits into a matching groove in a frame or on the housing. The curved lens elements tend not to separate with temperature changes and the like and focus and display quality are maintained.

Abstract: A track ball mechanism is mounted on a cantilevered portion of a printed circuit board that is supported in a housing. A switch, mounted on the opposite side of the cantilevered portion of the printed circuit board, is operated when it engages a boss on the housing. The ball of the track ball mechanism is operated by a human digit in the usual manner and the switch is operated when pressure on the ball deflects the cantilevered portion of the printed circuit board which causes the switch to engage the boss. The boss is supported on a cantilevered portion of the housing which deflects under excessive loading to protect the switch and track ball mechanism.