periodically exhale through their nose to equalise the internal pressure of the mask with that of the surrounding water. Swimming goggles are not suitable for diving because they only cover the eyes and thus do not allow for equalisation. Failure to equalise the pressure inside the mask may lead to a form of barotrauma known as mask squeeze. Masks tend to fog when warm humid exhaled air condenses on the cold inside of the faceplate. To prevent fogging many divers spit into the dry mask before use, spread the saliva over the inside of the glass and rinse it out with a little water. The saliva residue allows condensation to wet the glass and form a continuous wet film, rather than tiny droplets. There are several commercial products that can be used as an alternative to saliva, some of which are more effective and last longer, but there is a risk of getting the anti-fog agent in the eyes. Dive lights Water attenuates light by selective absorption. Pure water preferentially absorbs red light, and to a lesser extent, yellow and green, so the colour that is least absorbed is blue light. Dissolved materials may also selectively absorb colour in addition to the absorption by the water itself. In other words, as a diver goes deeper on a dive, more colour is absorbed by the water, and in clean water the colour becomes blue with depth. Colour vision is also affected by the turbidity of the water which tends to reduce contrast. Artificial light is useful to provide light in the darkness, to restore contrast at close range, and to restore natural colour lost to absorption. Exposure protection Protection from heat loss in cold water is usually provided by wetsuits or dry suits. These also provide protection from sunburn, abrasion and stings from some marine organisms. Where thermal insulation is not important, lycra suits/diving skins may be sufficient. A wetsuit is a garment, usually made of foamed neoprene, which provides thermal insulation, abrasion resistance and buoyancy. The insulation properties depend on bubbles of gas enclosed within the material, which reduce its ability to conduct heat. The bubbles also give the wetsuit a low density, providing buoyancy in water. Suits range from a thin (2 mm or less) "shortie", covering just the torso, to a full 8 mm semi-dry, usually complemented by neoprene boots, gloves and hood. A good close fit and