A method is disclosed for delivering a quantity of a substance into an eye. By applying compressed gas into an actuation chamber, a separating member connected to a lock mechanism and a plunger is displaced. The plunger displaces the quantity of the substance from a dispensing member and into an eye. The plunger is retained in an extended position so that the quantity of the substance is not retracted back into the dispensing member when the dispensing member is removed from the eye.