GLASS, ULTRAVIOLET RAY TRANSMITTING.
Glasses having the property of transmitting radiations of shorter wave-length than occur in the visible spectrum, that is, shorter than 400µ2 (the sign µµ representing 10,0,1,00. cm.) have been known for many years. Fused silica or quartz provides a glass transmitting rays down to 19oµ,1 while in 1907, E. Zschim mer prepared in small quantities a number of glasses having transmissions varying from 186µµ to 280/4/2. As the most active rays which affect a photographic plate are those of short wave length lenses which transmit well in the ultra-violet are advan tageous in camera work, and Schott, at Jena, on the basis of Zschimmer's work, introduced such a glass under the name of Uviol, although only made in small quantities. Especially is good transmission of importance for stellar photography and W. D. Haigh showed, at the Optical Convention in London in 1926, that British optical glasses in this respect could claim some advantage.
The great stimulus to the development of glass transmitting the shorter ultra-violet rays has been given in recent years as the result of the study by Bernard at Samaden and Rollier at Leysin of the beneficial effect of high Alpine sunlight on tubercu losis patients and of the subsequent employment for combating diseases of undernourishment, such as rickets, of electric-arc lamps emitting short ultra-violet radiations. The rays of thera peutic value have a wave-length about 300/2/2. Owing to absorp tion by the atmosphere, the shortest solar rays available, even in the Alps, are 295µ2, but ordinary window glass seldom trans mits the rays shorter than 310µ1. The question naturally arose as to the possibility of preparing a window glass to transmit the therapeutic rays. F. E. Lamplough in England first successfully did so in 1925, the glass, known as Vita-glass, being now made in large quantities. Fused boric oxide is, like quartz, transparent even to very short rays and Vita-glass resembles ordinary window glass except that it contains about 2% boric oxide in addition and is of low iron oxide and titanium oxide content. The pres ence in glass of iron oxide in the ferric condition greatly diminishes transparency to the shorter ultra-violet rays and suc cess in manufacture depends largely on their exclusion. Lead glasses absorb the ultra-violet rays and are, therefore, unsuit able. Several glasses have now been put on the market of which the most remarkable is the American glass known as Corex, although it is insufficiently weather-resisting to be of general use.
The following measurements made in the Department of Glass Technology, Sheffield, on samples 2 mm. thick, indicate the trans mission in the region of therapeutic value of a number of speci mens of the glasses now available: After short service the transmission of most such glasses diminishes somewhat and then remains constant. (W. E. S. T.) See also SUNLIGHT TREATMENT.