334 MICRO-ORGANISMS IN WATER 



have been little more than confirmations or additions 

 of detail to their classical work. These investigators 

 used sterilised Pasteur solutions, into which either a 

 minute quantity of a liquid swarming with bacteria was 

 introduced, or infection was secured by exposing the 

 tubes for a short time to the air. These infected test- 

 tubes were allowed to stand near a window with a south- 

 east aspect day and night for days, weeks, and even 

 months, direct sunlight reaching them for only a few 

 hours daily. As a control, some of these tubes were 

 carefully covered up in lead paper in order to expose 

 them to the same conditions as the others, the light only 

 being excluded. According as the solutions became tur- 

 bid or remained clear it was inferred that the organisms 

 had or had not grown, but microscopical examination 

 was also frequently resorted to in order to determine 

 this point more certainly. As a result of their observa- 

 tions, they stated that under favourable conditions ex- 

 posure to light entirely prevented the development of 

 these 'bacteria, whilst under less favourable conditions 

 their growth was only thereby retarded ; moreover, 

 that the direct rays of the sun were found to act most 

 prejudicially, although diffused daylight had also a 

 damaging effect. Experiments were also made and 

 published later 1 to ascertain whether any particular, and 

 if so what, rays of the spectrum acted more immically 

 than the rest upon bacteria, and it was found that bac- 

 teria exposed to the red and orange-red rays had their 

 development delayed, whilst the blue and violet rays 

 entirely prevented their growth. 



Downes and Blunt explain this action of light as due 

 to the gradual process of oxidation which is induced by 

 the sun's rays in the presence of oxygen, which process, 



1 ' On the Influence of Light upon Protoplasm,' Proc. Hoy. Soc. vol. 

 xxviii., 1878, p. I'M). 



