THE object of the present paper is to give the general characteristics of certain newly-developed glasses sometimes used for protecting the eye from radiant energy, especially the infra-red or so-called heat rays. Because of the difficulty in reproducing the same color in different melts, no attempt is made to give specific data on the transmission for a given thickness of glass. In order to obtain exact data it is necessary to examine samples from each melt.
These data are representative of an extensive group of glasses available for protecting the eye from (i) the ultra-violet, (2) the visible, and (3) the infra-red rays.
For protecting the eye from ultra-violet light, black, amber, green, greenish-yellow, and red glasses are efficient. Spectacles made of white glass afford some protection from the extreme ultra-vio,let rays which come from mercury-in-quartz lamps and from electric arcs between iron, copper, or carbon. The vapors from these arcs emit but little infra-red radiation in comparison with the amount emitted in the visible and in .the ultra-violet.
For shielding the eye from infra-red rays, deep black, yellowish-green, sage-green, gold-plated, and bluish-green glasses are efficient. For working near furnaces of mo.lten iron or glass if considerable light is~ needed, a light bluish-green or sage-green glass is efficient in obstructing the infra-red rays. For working molten quartz, operating oxy-acetylene or electric welding apparatus, or other intense sources of light, it is important to wear the darkest glasses one can use, whether black, green (including gold-plated glasses), or yellowish-green, in order to obstruct not only the infra-red but also the visible and the ultra-violet rays.
Data are given showing that of the infra-red rays emitted by a furnace heated to IOOO to i IOO ยฐ C. ( I ) about 99 per cent. are obstructed by gold-plated glasses, (2) about 95 per cent. by sagegreen or bluish-green glasses, (3) about 8o per cent. by very deep black glasses, and (4) about 6o per cent. by greenish-yellow glasses.
At higher temperatures these data would be somewhat different, but not sufficiently so to. modify the rough estimates dealt with in this paper.
* Technologic Paper No. 93.