426 Prof. R. W. Wood on Light Scattering by Air 



to take in the calculations. For example, the six miles of air 

 are considered as illuminated by sunlight of the full intensity 

 which it has in space. The light loses intensity as it 

 penetrates the air, and is reduced to about one half of its 

 value when it reaches sea-level. On the other hand, the 

 scattering power of the lower atmosphere appears to be 

 abnormally high, due to the presence of foreign matter, and 

 there is in addition secondary scattering; there is as well 

 probably some true absorption in the lower air. These effects 

 compensate to a certain extent, and on this account it seemed 

 best to consider the full intensity of the sunlight available 

 for the production of scattering, in the case of the sky. 



It would be far better to make the experiment on the top 

 of a high mountain, or even at one of the mountain obser- 

 vatories, and with the data given as to the dimensions and 

 disposition of apparatus, the whole thing could be done in a 

 day or two. The easiest way to make the slit on the disk 

 is to paste two strips of very thin black paper on a microscope 

 cover-glass and then paste the whole over a larger slit cut in 

 the pasteboard disk. When the proper width has been found 

 the cover can be detached and the slit width measured with 

 the microscope. This was the method adopted in my summer 

 laboratory with very limited facilities. An adjustable slit 

 would be more convenient of course. 



The colour match was very perfect, which alone indicates 

 that the light of the blue sky comes chiefly from the air 

 molecules; for, as will appear presently, the light scattered 

 by the foreign matter in the lower atmosphere is yellowish 

 in comparison with the colour of the clear sky. 



It is perhaps open to question whether we are justified 

 even in considering the sky illumination as represented by 

 the number of molecules in the line of sight (or in other 

 words, the thickness of the homogeneous atmosphere) mul- 

 tiplied by the intensity of the illumination. 



The relation holds undoubtedly for small thicknesses of 

 dust-free air, but Abbot's observations indicate that it does 

 not hold at all for the distances concerned in producing the 

 sky light. 



For example, he finds that with the sun at an altitude of 

 46°, the sky 3° above the horizon is less than double the 

 brightness of the sky at 57°, though the mass of air 

 under observation in the line of sight for the sky near the 

 horizon is thirteen times greater than in the case of the sky 

 at an altitude of 57°. He shows conclusively that the intensity 

 of the scattered light increases rather slowly in comparison 

 with the increase in the number of the scattering molecules. 



