WHENEVER light is produced, invisible W radiations-ultra-violet (U.V.) or infra-red (I.R.) are, almost without exception, produced at the same time. Glasses which are transparent to the visible rays are not necessarily transparent to either the ultra-violet or infra-red; and similarly, opacity to one set of radiations does not imply opacity to the other two sets. In order to control these invisible rays, therefore, it is necessary to have ranges of glasses for transmitting and for absorbing these various bands of rays.