410 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



of the scattered light. As larger particles grow during the process of 

 nucleation or are present as dust, the blue gets weak and whitish 

 from the scattering of white light. Near the horizon, where the light 



5 1 



X in ft /A X 10° 



FlGURE 1. 



traverses a long reach of atmosphere and coarser dust is common, one 

 gets the familiar weakening of the sky blue. 



The process of increasing nucleation, which results in cloud formation 

 and frequently in subsequent rain, can be followed very closely by the 

 polariscope. A fall in polarization, particularly when the spectroscope 



