When ultraviolet rays from such a source as a mercury vapor or carbon arc are intercepted by a special screen of glass containing nickel oxide, which is almost opaque to ordinary visible light rays, a certain group having a wave-length of about 3,650 angstrom units passes through the screen. This causes fluorescent bodies to give off a light varying with the color of the material, such as red from eosin, yellow from zinc sulphate, blue from quinine sulphate or violet from sodium salicylate.
This so-called Wood light, from the name of the inventor of the glass, has been used for some time in medicolegal work to detect minute spots of blood, semen and other organic fluids, which fluoresce characteristically when present even in minute quantities.
The use of the Wood light in dermatologic diagnosis is to be distinguished from the method which André Broca used—that of passing light through cobalt