Report on the Bacteriology of Water. 387 



that the fundamental physiological function affected by the light is 

 not growth, but nutrition constructive metabolism ; and this would 

 coincide very well with what Elfving found to be the case in macro- 

 scopic cultures of fungi. 



7. However, the possibilities are not exhausted by the above. 

 There are several points which suggest that the destructive light 

 action may take place on the enzymes which the living cell forms and 

 excretes. If Green is right in concluding that the more refrangible 

 rays* are destructive to the action of enzymes outside the cell, it may 

 be that we have here the key to the mystery, and that the cells 

 gradually die of inanition from inability to render their food materials 

 assimilable. Still the experiments throw no light on whether such 

 action takes place outside or inside the cell,f though, perhaps, the 

 results with spores support the latter idea rather than the former 

 one. 



8. Whatever the light action consists in, it is evidently exerted by 

 the more refrangible rays, and is the more pronounced the more 

 intense the light, or with feeble lights the greater the . proportion of 

 these rays there are in it. 



9. The evidence goes to show also, that the rays at the other end 

 of the spectrum, and especially the heat rays in the red and below, 

 co-operate in the light action in question (or, possibly, sometimes 

 antagonise it) in various ways. The probability is that with a given 

 moderate intensity of light, such as occurs in ordinary daylight, the 

 damaging effect of the blue- violet rays on growing cells is dependent 

 on the temperature. If the temperature is about the optimum, the 

 protoplasm, working at its best, seems able to resist perhaps even 

 nndo the damage; if the temperature is far removed from the 

 optimum, however, the injurious action of the light rays is cumulative, 

 and results in more or less rapid retardation of growth, and eventual 

 death. This is, no doubt, true, whether the protoplasm fails to com- 

 bat the injury because too passive, as at low temperatures, or because 

 over-stimulated and too active, as at high temperatures. Taking into 

 account all we know, however, it seems improbable that the organism 

 <jan resist the bactericidal action of the more refrangible rays at any 

 temperature, if those rays are relatively abundant in an intense light. . 



* Green " The Influence of Light on Diastase " (' Annals of Botany,' 1894, pp. 

 370373). 



t It is not inconceivable that when the enzymes begin their work in the food 

 material, the products of their action are easily oxidised. Indeed some experi- 

 ments point to the probability of this, and suggest that broth with enzymes in it is 

 susceptible to some destruction. 





