52 SCIENCE PROGRESS 



certainly had little tendency to produce the more severe acute 

 results which immediately attract attention. There is good 

 reason, however, to think that they are by no means innocuous. 

 The problem can only be solved conclusively by experiments on 

 animals and many of great value have been recorded, though 

 they are not altogether confirmatory and much work yet remains 

 to be accomplished. 



We must first inquire what happens to ultra-violet rays 

 when they pass into an eye. If we look at a spectrum in an 

 absolutely dark room and screen off the luminous portion, we 

 shall find that the ultra-violet part near the violet end is not 

 completely invisible ; it has a faint greyish luminosity. This 

 may be due either to the possibility that these rays are capable 

 of stimulating the percipient elements of the retina and so pro- 

 ducing visual impressions or to the possibility that certain parts 

 of the eye are capable of degrading the ultra-violet rays into 

 rays of longer wave-length which are faintly luminous. This 

 degradation of ultra-violet rays, which is a property possessed 

 by various substances, is called "fluorescence." Since some of 

 the structures of the eye undoubtedly fluoresce when exposed 

 to these rays of short wave-length, the second alternative must 

 be accepted as the more probable. The structure which shows 

 this property most is the crystalline lens. If ultra-violet rays 

 pass into the pupil, the lens is seen to fluoresce strongly. The 

 same occurs if the eye is exposed to the emanations of radium. 

 Bodies which fluoresce do so by absorbing the ultra-violet rays. 

 Their absorption by the lens is strikingly shown by placing 

 an excised lens on the skin and exposing both to strong ultra- 

 violet rays. The skin becomes "sunburnt" everywhere except 

 where the lens lies, the part thus protected remaining normal 

 or much less affected. The retina also fluoresces, though much 

 less. It has been shown experimentally that if light rich in 

 ultra-violet rays is allowed to pass into the eye for a considerable 

 time the lens loses its power of absorbing these rays. They 

 then pass through almost unimpeded and are able to effect 

 any deleterious consequence of which they are capable upon 

 the delicate structures of the retina. Absorption means a 

 utilisation of energy which must be associated t with some 

 chemical or other change. It is therefore not unlikely that pro- 

 longed exposure to ultra-violet rays may produce pathological 

 changes in any structure of the eye which normally] absorbs 



