The imaginative and experimental material of this book was inspired by the work of Gurwitsch the Russian investigator who found that certain cells in their growth emit ultraviolet rays. The author thinks that individual variation in susceptibility to rickets may depend on the individual organism's ability to produce radiation of this sort. He has devised a method of determining whether ultraviolet radiation is emitted by the blood. He finds that this phenomenon was present in most of the newborn infants studied who were born at full term but absent in the premature infants. It may be developed by treatment with sunlight or with ultraviolet radiation. According to the author, such treatment is superior to supplying vitamin D by mouth.