Although in German spelling Grenz, like all nouns, begins with a capital letter, its English derivative is spelled with a lowercase g. Very soft x-rays, closely allied to the ultraviolet rays in their wavelength (i.e., relatively long) and in their biologic action upon tissues; they are produced by a specially built vacuum tube with a hot cathode operating from a transformer delivering not more than 8 kw. [Ger. Grenze, borderline, boundary]