<p>A device for preventing the observing objective lens window of an endoscope from collecting moisture by directing a small flow of air over the front surface of the observing objective lens window to form a layer of air which has the same temperature as the ambient atmosphere thereby stabilizing the temperature of the window. A control valve is provided which shuts off the flow of air in the event that the pressure within the body cavity in which the endoscope is inserted becomes too high due to the flow of air. A first embodiment of the control valve includes a cylinder and piston arrangement in which at a first position of the piston air is allowed to flow through the tube to the endoscope end. At a second position of the piston, air is vented directly to the atmosphere. In a second embodiment, a lateral hole is provided in the piston with the area of the hole and hence flow of air controlled by a screw the position of which determines the area of the lateral aperture.</p>
