Recent investigation has established the fact that white rats can be regularly protected against rickets by means of sunlight or artificial rays produced by the mercury vapor quartz or the carbon arc lamp.1All these sources of light emit not only visible radiation, but also ultraviolet rays of various wave lengths. In a previous paper, Hess, Unger and Pappenheimer drew attention to the fact that rats which were confined in an enclosure of ordinary window glass (3 mm. thick) failed to be protected by sunlight, as by this procedure the effective rays were filtered out of the spectrum. In a subsequent paper,2it was shown that a filter of ordinary window glass (Corning), which transmitted ultraviolet waves as short as 334 millimicrons, intercepted the effective rays from the mercury vapor lamp, indicating that not all ultraviolet rays possess this property, but that it is associated only with waves