Experimental evidence which can shed light on the activity of the tubercle bacillus in tissue and its resistance to modern methods of therapy is of value. The advent of the use of heliotherapy has marked a great advance in the physiotherapy and control of many forms of local tuberculosis. A particularly significant addition to the medical armamentarium was the air-cooled quartz mercury vapor lamp, commonly spoken of as the ultraviolet lamp or the Alpine sun lamp.
In the world at large, sunlight is the great destroyer of bacteria, and even in the animal body, if the effects of its indirect action are considered, it must be given first place. Sunlight in many countries, however, is available for only short periods of the year, and even then is an uncertain therapeutic and experimental agent, due to weather conditions. This has limited its value and led to the creation of substitute sources