6 The Influence of Light on Bacteria. 



The doubt was whether the more rapid development in the 

 covered bottle was due to the protection from the light, or 

 to the more uniform temperature preserved by the paper 

 wrapping. I therefore varied the conditions in the following 

 way : — 



Exp. III. — Three small thin phials were half filled with 

 inoculated solution, and suspended just inside of a window, 

 as in the last experiment, on 6th March at noon, the day 

 being bright and warm. One of them was not protected at 

 all from the sun ; the second was shielded from its rays by 

 a small piece of thin white paper put between it and the 

 glass of the window ; while the third was more fully pro- 

 tected by means of a larger piece of thick brown paper. 

 The 7th was bright and very hot ; the 8th warm, but cloudy 

 after the morning. On the 9th, at 9 a.m., both the protected 

 bottles showed slight opalescence, which steadily increased, 

 though without noticeable difference in them. Only on the 

 11th was there slight cloudiness in the exposed bottle, which 

 became distinct on the 14th ; and on the 19th, after several 

 very clear, hot da}< s, it was quite milky and crusted. It may 

 seem that the influence of the direct rays of the sun in re- 

 tarding development is here quite apparent. That the 

 retardation may in part have been owing to that I am not 

 prepared absolutely to deny ; but it is also evident that the 

 unprotected bottle was also exposed during the day to a 

 higher temperature than the others, and possibly also to a 

 .•slightly lower temperature during the night, and thus to 

 greater fluctuations, both upwards and downwards toward 

 unfavourable extremes. I have not been able to devise any 

 arrangement whereby a nearer approach than in this case 

 could be got to uniformity of temperature with varying 

 intensities of light. I claim, however, to have again shown 

 clearly, in opposition to the conclusions of Messrs. Downer 

 and Blunt — 



(1) That the brightest diffused light is not inimical to the 

 development of bacteria ; and (2) that full exposure to the 

 sun's rays is not destructive to bacteria or their germs, 

 when precautions are taken, as by suspension, against ex- 

 posure to too high degrees of temperature. 



I cannot add that such exposure to the sun's rays in no 

 way retards development, but I must express the conviction 

 that retardation may generally with equal propriety be 

 ascribed to extremes of temperature associated with the 

 insolation. 



