THE LUMINOUS EFFICIENCY OF ILLUMINANTS 551 



The peculiar physiological and psychical effects of coloured 

 light hardly fall within the scope of this article but may be 

 mentioned briefly. There is a popular impression (supported it 

 would seem by scientific researches) that the red end of the 

 spectrum is exciting and stimulating whilst the green and blue 

 rays have a sedative or even depressing effect. Thus the colour 

 red, apart from its well-known irritant effect on animals, has 

 been habitually associated with violent ideas such as war, 

 revolution, etc. 



According to some medical authorities patients subject to 

 melancholia are braced and favourably influenced by sitting in a 

 red room, whilst excitable maniacs are often soothed by confine- 

 ment in green and blue surroundings. In some oriental countries 

 permanent enfeeblement of the mental faculties of inconvenient 

 political prisoners is said to have been deliberately produced by 

 prolonged exposure to blue light. In a concentrated form blue 

 light is also said to produce local anaesthesia of the skin. 



All these are matters which are mainly of interest to the 

 physiologist and seem to require more substantial confirmation. 

 But from the physical side it may be pointed out that the 

 existence or non-existence of such phenomena could doubtless 

 be much more readily studied provided we were able to secure 

 pure red and blue light in large quantities. Meantime it is of 

 interest to mention, in passing, that somewhat analogous experi- 

 ments on the effect of coloured light on plants were carried out 

 some years ago by Camille Flammarion. He showed that red 

 light tends to produce abnormally rapid growth while blue light 

 retards development. Also one of the sensitive mimosa plants 

 was rendered comatose and unresponsive when placed in blue 

 light but showed quite unusual excitement under red rays. The 

 effect of coloured light on plants is thus another subject to which 

 the remarks made above might be applied. Steinmetz has 

 attempted to generalise on its influence by suggesting that the 

 comparatively slow visible red rays and heat rays, by producing 

 resonance with complex groups of atoms, are favourable to 

 growth and life, while the more rapid blue rays do not readily 

 produce any such quickening effect. Ultra-violet rays (which in 

 excess are known to be prejudicial to plant life), on the other hand, 

 are assumed to produce resonance of individual detached atoms 

 or electrons, breaking down the molecule and leading to death. 



Allusion has already been made to the generation of ultra- 



