In the Finsen light treatment electric light is used for a very different purpose. The spectrum of white light consists of the colours red, orange, yellow, green, blue, indigo, and violet. Just beyond the violet end of the spectrum are the ultra-violet rays. Ultra-violet light consists of waves of light which are so short as to be quite invisible to the eye, and Dr. N. R. Finsen, a Danish physician, made the discovery that this light is capable of destroying bacterial germs. In the application of ultra-violet rays to medical work, artificial light is used in preference to sunlight; for though the latter contains ultra-violet light, a great deal of it is absorbed in passing through the atmosphere. Besides this, the sun sends out an immense amount of radiant heat, and this has to be filtered out before the light can be used. The usual source of light is the electric arc, and the arc is much richer in ultra-violet rays if it is formed between electrodes of iron, instead of the usual carbon rods. The light, which, in addition to the ultra-violet rays, includes the blue, indigo, and violet parts of the spectrum, is passed along a tube something like that of a telescope, and is focused by means of a double lens, consisting of two separate plates of quartz. Glass cannot be used for the lens, because it is opaque to the extreme ultra-violet rays. A constant stream of water is passed between the two plates forming the lens, and this filters out the heat rays, which are not wanted. In some forms of Finsen lamp an electric spark is used as the source of light, in place of the arc. The most important application of the Finsen light is in the cure of the terribly disfiguring disease called lupus. This is a form of tuberculosis of the skin, and it is produced by the same deadly microbe which, when it attacks the lungs, causes consumption. In all but extreme cases the Finsen light effects a remarkable cure. A number of applications are necessary, each of half an hour or more; and after a time the disease begins to disappear, leaving soft, normal skin. The exact action of the light rays is a disputed point. Finsen himself believed that the ultra-violet rays attacked and exterminated the microbe, but a later theory is that the rays stimulate the tissues to such an extent that they are enabled to cure themselves. As early as the year 1899 Finsen had employed his light treatment in 350 cases of lupus, and out of this number only five cases were unsuccessful.