Electroless plating is performed to deposit conductive materials on work pieces such as partially fabricated integrated circuits. If the internal pressure of a reactor increases, vertical growth becomes faster, and internal crystal defects are reduced while many fine pits are generated in view of outer appearance. The indexing mechanism is particularly useful with a plug that lands in a seat to impart an axially directed force on the mechanism before passing through the seat. The commanded radiance and measured radiance are compared, and drive currents for the laser diodes are adjusted to provide luminance and color balance tracking. The driving force for driving the vehicle is controlled based on the calculated target driving force in dependence on the determined traveling condition of the vehicle. The apparatus is useful in accumulating articles transported on a main conveyor in a first-in, first-out fashion.