160 



DISCOVERY 



ultra-violet, there are certain sources which are rela- 

 tively rich in this radiation. Amongst these may be 

 mentioned certain arc lamps having carbons cored with 

 metallic materials which impose ultra-violet lines over 

 the ordinarj' spectrum. Such sources have been used 

 in kinema studios, where the richness in ultra-violet 

 light is useful on accomit of the enhanced actinic 

 effect, and cases of temporary injury to eyesight of 

 actors have been recorded. A committee operating 

 under the Ministry of Health recently considered this 

 matter very fully, and came to the conclusion that 

 the liability to injury could be easily avoided if the 

 sources were properly screened. Experts seem to 

 agree that the use of very powerful unscreened lights 

 is not necessary, except possibly in rare cases, in the 

 kinema studio ; and that the most pleasing effects are 

 obtained when the light is softened and diffused by 

 reflection or transmission through translucent materials 

 giving conditions approaching more nearly to daylight. 



Arc-welding is another process where injury to the 

 eye through ultra-violet light must be guarded against. 

 In this case the richness in ultra-violet is due to the 

 same cause — the mingling of metallic materials giving 

 strong ultra-violet spectra, with the carbon. When a 

 carbon arc is used to weld or cut iron plates, the spectrum 

 of iron, containing many prominent lines in the ultra- 

 violet, is superimposed over the continuous spectrum 

 of the glowing carbon. Operators must accordingly 

 protect their eyes and skin by the use of goggles and 

 masks. 



So far as can be ascertained, the great majority of 

 injuries that have been caused by ultra-violet energy 

 are transient. The sufferer frequently feels no ill- 

 effects at the time, but wakes up in the night to find 

 his eyes in a most inflamed and painful condition. 

 This passes off in due course. But it is conceivable 

 that prolonged exposure to excess of ultra-violet light 

 may have serious permanent effect. For example, 

 cataract has been attributed to this cause, and it is a 

 fact that albuminous transparent material can be 

 almost instantaneously coagulated by exposure to 

 these rays. An interesting research by Burge ' leads 

 to the suggestion that certain conditions of health, 

 leading to an undue proportion of salts of calcium, 

 magnesium, and sodium in the eye-lens, may accentuate 

 [ the tendency of ultra-violet rays to cause cataract. 

 This may explain why a few persons frequently 

 exposed to these rays suffer, while the great majority 

 escape. 



It should not be assumed that ultra-violet light is 

 necessarily prejudicial. In this case, as in many 

 others, what is harmful when indiscriminately applied 

 may be beneficial when used with discretion. Exposure 

 of the body to ultra-violet light has proved of great 



' Electric World, April lo, 191.5. 



value in the treatment of various skin diseases ; the 

 rays which inflame the living tissue may kill a parasitic 

 growth. Of the special sources used for this purpose, 

 interest attaches to the mercury vapour lamp with 

 a quartz glass tube, probably the richest of all artificial 

 sources in ultra-violet light. Ladenburg has estimated 

 that as much as 30 per cent, of the radiation of lumines- 

 cent mercury is located in the ultra-violet light, and 

 the quartz tube allows these rays to pass unimpeded. 

 But in addition to these therapeutic uses, the exposure 

 of the body to the ultra-violet energy derived from 

 the sun is believed to be beneficial to the adult, while 

 in the case of children it has a special value. Several 

 infantile ailments are closely associated with access of 

 light. Rickets is now regarded as a " disease of dark- 

 ness." It was formerly thought that poor diet was 

 the determining factor. Then it was found that the 

 disease was contracted mainly by children in large 

 towns who were little exposed to sunlight ; also that 

 in some cities in India the children of the very poor 

 who lived on an inferior diet but constantly played in 

 the open air did not contract the disease, whereas the 

 children of wealthy Hindoos who were well fed but 

 lived an indoor, secluded life were very prone to it. 

 Sir Henry Gauvain, in recently referring to these facts, 

 mentioned that exposure to sunlight formed a regular 

 feature of the treatment in the Treloar Cripples' 

 Homes at Hajding Island, and it has recently been 

 found that exposure to artificial sources rich in ultra- 

 violet light can advantageously replace sunlight during 

 vmfavourable weather. 



The Fading of Colours 



Let us now turn to some effects of ultra-violet energy 

 on inanimate things. One of the most striking effects 

 due to this form of radiation is the fading of colours. 

 The ultra-violet rays are potent in causing chemical 

 action, of which photography has already been men- 

 tioned as a familiar example. The fading of objects 

 when exposed to sunlight is well known. In certain 

 cases, for instance in the case of priceless and unique 

 specimens in museums, it forms a serious problem. 

 Some interesting experiments on this point were 

 recently summarised by Sir Sidney Harmer, Director 

 of the Natural History Section of the British Museum, 

 in a lecture before the Royal Society of Arts. In 

 general natural colours are more permanent than 

 artificial pigments. Artists have to be discriminating 

 in their use of colours, for there are many pigments 

 which fade appreciably after only a few days' exposure 

 to strong sunlight : an extreme instance of instability 

 is to be found in the purple copying ink used on 

 ordinary typewriter ribbons, which fades noticeably 

 after a few hours' exposure, and ultimately may be 

 made to disappear entirely. Even delicate natural 



