50 SCIENCE PROGRESS 



effectual method of cure. We are here, however, specially- 

 concerned with prevention and need not dwell on these more 

 obscure problems. 



There is no doubt that snow-blindness and ophthalmia 

 electrica are caused by the action of ultra-violet rays. If 

 sunlight is split up into its component parts by being passed 

 through a glass prism a spectrum is obtained. If this is 

 focussed on a screen by lenses, a sharply defined band consist- 

 ing of all the colours of the rainbow is seen. The part of the 

 band on the side of the base of the prism, i.e. the part which is 

 least refracted, is red ; the other end is violet. The various 

 colours are due to differences in the wave-length of the light, 

 the extreme red end having a wave-length of 766 /i/i (millionths 

 of a millimeter), the extreme violet 397 pp. If the spectrum is 

 photographed it is found to be much longer on the violet side 

 than that seen by the eyes. There are, therefore, rays beyond 

 the violet end which are capable of producing chemical changes 

 in the sensitive plate. These are called the ultra-violet rays. 

 They may be made visible by placing a uranium glass in their 

 course, when the uranium salt which impregnates the glass is 

 said to " fluoresce." If a prism and lenses made of quartz instead 

 of glass are used to form the spectrum the ultra-violet part 

 is still longer. Glass, therefore, has the property of absorbing 

 part of the ultra-violet rays. It is found that a foggy atmosphere 

 has much the same effect as glass, so that sunlight in clear high 

 altitudes contains far more ultra-violet rays than elsewhere. 

 Moreover, snow surfaces and the surfaces of sheets of water re- 

 flect ultra-violet rays without causing any appreciable absorption. 

 The eyes are protected to a large extent from direct sunlight by 

 the overhanging brows and headgear. They are unprotected 

 from the reflected light off the ground surface. Ultra-violet rays 

 are not only responsible for these inflammations of the eye 

 but are also the cause of so-called sunburn. If the skin be 

 exposed sufficiently long to the ultra-violet rays only of the 

 spectrum it becomes " sunburnt." Similarly, it has been shown 

 experimentally that conjunctivitis exactly similar to snow- 

 blindness or ophthalmia electrica is caused by exposure of the 

 unprotected eyes of animals or men to intense ultra-violet rays. 



In seeking the cause of glass-makers' cataract it is obvious 

 that the question of heat must be taken into consideration. 

 This is the more forcibly impressed upon us by the fact that 



