EFFECTS OF BRIGHT LIGHT UPON THE EYES 51 



workers in certain forms of glass manufacture appear to suffer 

 little or not at all from the complaint. These men are engaged 

 in the manufacture of flint glass bottles, pressed glass articles, 

 and so on and the heat of the furnaces is much less than in 

 ordinary bottle making. If heat, however, were an important 

 factor, we might expect iron workers to suffer in a similar 

 manner but such is not the case. Moreover, it is difficult to 

 imagine that heat can penetrate far into the eye when we 

 consider that water is a bad conductor and that the space 

 between the cornea and the crystalline lens is filled with watery 

 fluid. In fact, the lens is admirably protected from heat by a 

 water bath. Cases, too, occur in which molten metal at an 

 extremely high temperature is splashed into the eye without 

 causing the development of cataract. If, therefore, such intense 

 degrees of heat as those under consideration fail to produce 

 deleterious effects upon the deeper structures of the eye, it is 

 probable that we may with safety eliminate this factor as of 

 any importance when we come to deal with sources of illumina- 

 tion in which the output of heat is incomparably less. 



It is a striking fact that the attempt to obtain greater 

 efficiency in artificial methods of illumination has resulted 

 almost invariably in the production of sources of light which 

 are extremely rich in ultra-violet rays. Possibly the belief that 

 ultra-violet light has beneficial effects in the treatment of certain 

 diseases of the skin may have influenced the trend of modern 

 inventions. Thus it has led to the invention and development 

 of the Finsen lamp. Here the immediate aim has been to enrich 

 the light with ultra-violet rays by impregnating the carbons of 

 the arc light with various metals. Experiments in this direction 

 have demonstrated that there is in some cases a marked con- 

 comitant increase in luminosity as measured by the photometer 

 and this aspect has attracted the attention of illuminating 

 engineers and lamp manufacturers. 



As has already been mentioned, exposure for any consider- 

 able length of time to light rich in ultra-violet rays causes 

 u sunburn " and intense conjunctivitis. What is at present 

 unknown is if still more prolonged exposure to much weaker 

 doses of ultra-violet rays has any deleterious effects upon the 

 eyes ; and if so, which of the ultra-violet rays, those nearer the 

 violet end of the spectrum or those more remote, have the 

 worse effects. These weaker doses spread over a long period 



