250 THE INFLUENCE OF LIGHT ON LIFE. 



These references are perhaps sufficient to show that different 

 coloured lights influence animals in different ways. 



It is scarcely necessary to mention, before we pass to another 

 matter, the influence of darkness in inducing sleep among so many 

 animals. Even when darkness comes on at unusual times — such 

 as during an eclipse of the sun — it has been noticed that the birds 

 ceased singing and, like many other creatures, went to sleep. 



I now wish to place before you a brief resume of some recent 

 investigations on the influence of light on certain micro-organisms 

 as set forth by Professor Percy Frankland. Although the effects 

 which have already been mentioned are interesting, yet the follow- 

 ing results of sunlight upon microbes are almost more interesting 

 and wonderful. Messrs. Downes and Blunt put some test-tubes, 

 containing a liquid which was swarming with bacteria, in such a 

 position that the tubes were exposed to the direct rays of the sun 

 for several hours daily. The result of this exposure was that when 

 the light was most favourable the development of the bacteria was 

 entirely prevented, and when the light was not so strong the growth 

 of the microbes was only retarded. In tubes which had been 

 screened from the light, the bacteria were found to have increased 

 in numbers. The direct rays of the sun were proved to be most 

 effectual in destroying the organisms, but diffused daylight was 

 found to have a considerable damaging effect upon them. 



As a result of experiments which were made with a view of 

 finding out which rays were most active in bringing about such a 

 novel effect, it was ascertained that bacteria which were exposed to 

 the blue and violet rays had their growth entirely prevented, 

 whereas those which were exposed to the red rays of the spectrum 

 had their development delayed. It is worthy of particular notice 

 that the rays which exert the greatest destroying power upon the 

 bacteria are the same as those which we have just stated were 

 most obnoxious to the ants — indeed, the same rays which most 

 powerfully affect the photographer's plate. This destroying action 

 of light is said to be a process of oxidation^ as the action is 

 increased when the supply of oxygen is increased, and diminished 

 when the oxygen is diminished. For instance. Anthrax bacilli 

 which were exposed to the direct rays of the sun in the presence 

 of air, were killed in two and a-half hours ; whereas Anthrax 



