550 SCIENCE PROGRESS 



as a luminous point a long distance away, it might be better to 

 aim at producing the green rays. 



There are also cases in which it would apparently be 

 beneficial to use monochromatic light. Indeed in certain con- 

 ditions some of the rays produced by modern illuminants may 

 be actually prejudicial to the purpose in view. One instance of 

 this is afforded when light is used not to " create brightness " 

 but to " reveal detail." It is well known that the eye is not 

 achromatic, i.e. cannot bring to a focus rays from all parts of the 

 spectrum simultaneously. Ordinarily, we may suppose, the eye 

 accommodates itself to the intermediate region, whilst the red and 

 violet appear somewhat " fuzzy " and therefore spoil the definition 

 of the object. Recent experiments lead one to suppose that 

 acuteness of vision might be substantially increased by using 

 monochromatic light. It has also been suggested that red light 

 gives the best definition in the case of distant illuminated objects 

 whilst blue and green light are best for near vision. This at 

 least is the writer's experience but it appears that it may not be 

 universally true for all eyes. 



Another special use for coloured light is in connection with 

 decorative effects. At present our methods of producing coloured 

 light are based mainly on absorption and are therefore woefully 

 inefficient. In order to produce blue light, for example, we start 

 with a white light and then absorb and lose all but the blue, 

 which is probably but a fraction of i per cent, of the total 

 energy generated. Thus in obtaining pure blue light by this 

 means from an ordinary metallic-filament lamp, screened with 

 blue glass, only about i/5oooth of the energy actually given to 

 the lamp would be ultimately used. 



If, however, we could discover a means of generating only 

 light of the one colour we desired to use, such light might prove 

 to possess unexpected and valuable qualities. At present very 

 little is known about the effects of monochromatic light ; chiefly 

 because we have no ready means of producing it in reasonably 

 large quantities. 



As an illustration of the possibilities in this direction we may 

 take the well-known therapeutic qualities of the Finsen lamp. 

 Its curative effect is ascribed to the blue and violet rays which are 

 filtered out from the light yielded by a very powerful arc lamp ; 

 but it is only by the concentration and waste of a very large 

 amount of luminous energy that this effect becomes apparent. 



