the reflecting face of the mirror 10 as can be seen the water tube 12 is positioned above the air tube 13 and the tip of the tube 12 is level with the tip of the tube 13 and the lower edge of the tip of the tube 12 is in contact with the tube 13 the diameter of the tubes 12 and 13 is not critical but as an example of the bore of the water tube 12 is approximately 0 02 cms in diameter and that of the air tube approximately 0 1 cms the tip of the air tube 13 is approximately 1 4 inch away from the mirror 10 air under pressure is fed through the tube 13 water under pressure is fed through the tube 12 and is sucked smoothly away by the stream of air flowing from the tube 13 at the tip of the air tube 13 each drop of water is atomised into a very fine continuous spray which is carried along in the air stream and impinges on the lower edge of the face of the mirror 10 the fine spray spreads over the surface of the mirror to form a very thin continuous.