and the Blue Colour of the Sky. 431 



to the value which we are to assign to the intensity of the 

 sunlight effective in illuminating the five miles of homo- 

 geneous atmosphere, if we consider the intensity at the 

 earth's surface as 50 per cent, of its value in space. If we 

 make the same assumption as before, we should expect for a 

 very clear day 1000x1/2 = 5 per cent, of 26100x1 if the 

 atmosphere were uniform in composition. To have this 

 equation hold we must multiply the left-hand term by 2*64, 

 which means that the scattering power of the air close to 

 the earth's surface is about 2 6 times the average scattering- 

 power of the entire atmosphere. 



These results appear to be in good agreement with Abbot's 

 observations on atmospheric absorption (absorption in this 

 case to be understood as the removal of energy from the 

 primary beam of light by scattering). Abbot found that the 

 loss of intensity due to passage through the lower mile of the 

 atmosphere was equal to the loss suffered by passage through 

 the entire atmosphere above the first mile. He also found 

 that the intensity of the solar radiation for the middle of the 

 visible spectrum at the earth's surface was about 50 percent, 

 of its value iu space. If_, now, we consider the atmosphere as 

 tin ocean of air at standard pressure five miles in depth (the 

 O-mile homogeneous atmosphere), and if we consider the 

 percentage of foreign matter as constant throughout its mass, 

 each mile will remove 12 per cent, of the energy from the 

 light, and passage through the entire five miles will give a 

 residual intensity of 53 per cent, of its original value. This 

 12 per cent, we may call the average absorbing power of 

 one mile of the atmosphere, the quantity which we are to 

 compare with the scattering power of 1000 feet of surface air 

 observed with the dark cave. The facts of the matter are 

 that most of the foreign matter is in the lower mile. Con- 

 sidering the atmospheric ocean as divided into two layers, a 

 lower layer one mile in thickness and dust-laden and an 

 upper layer of four miles free from dust, an absorption of 

 -30 per cent, by each layer will be in close agreement with 

 Abbot's results. This absorption of 30 per cent, by the 

 lower mile is 2*5 times the average absorbing power of one 

 mile of the homogeneous atmosphere, and is in good agreement 

 with the observation that the scattering power of the air near 

 the ground was 2'G times the average scattering power of 

 the entire atmosphere. 



Demonstration Apparatus for the Laboratory. 



The phenomenon of light scattering by pure gases is of 

 such fundamental importance from a theoretical standpoint, 



