158 GENERAL PHYSIOLOGY 



levels, more in the morning than at night ; and Blichner deter- 

 mined that the light could penetrate water to the depth of a 

 meter or more, and that when bacteria were exposed at various 

 depths they were affected by the sunlight in inverse proportion 

 to the depth below the surface. The effect of the sunlight 

 varies with its intensity, and hence varies with the season of 

 the year, the time of day, and the condition of the sky. In 

 summer the action is several times as pronounced as in winter. 

 A great deal of discussion has occurred regarding the method 

 of the action of sunlight on the bacteria. At first it was 

 thought that perhaps it was due to heat, but it can be readily 

 shown, by interposing a screen, that the action takes place 

 entirely independent of the heat. It has been found that it 

 varies with the kind of light ; that is, if white light is broken 

 up into its component parts, certain parts of the spectrum are 

 much more effective than others. If a screen is placed between 

 the sun and the bacteria, which shuts out all but the less 

 refracted rays, such as may be done by the use of a screen 

 of potassium bichromate, it will be found that there is little or 

 no action ; that is, these rays are not effective. But if a 

 screen is interposed between the bacteria and the sunlight 

 which allows the more highly refracted rays to pass through, 

 such a screen, for instance, as one composed of an ammo- 

 niacal solution of copper sulphate, it is found that the 

 maximum effect is obtained. Or, if a spectrum is thrown on a 

 heavily seeded agar plate, it will be found that the greatest 

 effect is in the blue- violet rays, that it diminishes very quickly 



