The term quartz light as generally used refers to a mercury vapor light in a fused quartz vacuum tube, the mercury making the arc. Such a lamp is very rich in ultraviolet light; and, as quartz to a large degree is transparent to ultraviolet rays, these rays emerge from the lamp and can be utilized for therapeutic purposes. As to the nature of ultraviolet light: When sunlight is passed through a prism, it is split up into many rays. We are all familiar with the visible rays of color so produced, but at each end of the visible spectrum there are innumerable invisible rays. At one end of the visible spectrum we have the invisible infra-red of longer and longer wave lengths the farther we go beyond the visible red, while at the other end beyond the visible violet we have the invisible ultraviolet, of shorter and shorter wave length