174 Sir David Brewster on the Polarization 



parcels, become first of that bigness whereby such an azure must 

 be reflected"*. 



Professor Clausius considers the vapours to be vesicles or flat- 

 tened bladders, and ascribes the blue colour of the first order to re- 

 flexion from the thin pellicle of water. In reference to these 

 opinions the following facts are important : — 



(1) The azure colour of the sky, though resembling the blue of 

 the first order, when the sky is viewed from the earth's surface, 

 becomes, as observed by Mr. Glaisher in his balloon ascents, an 

 fe exceedingly deep Prussian blue " as we ascend to the height of 

 five or six miles, which is a blue of the second or third order. 



(2) The maximum polarizing angle of the atmosphere being 

 45°, is that of air, and not that of water, which is 53°. 



(3) At the greatest height to which Mr, Glaisher ascended, 

 namely, at the height of five, six, or seven miles, where the blue 

 is the brightest, " the air is almost deprived of moisture." 



Hence it follows that the " exceedingly deep Prussian blue " 

 cannot be produced by vesicles of water, but must be caused by 

 reflexion from the molecules of air, whose polarizing angle is 45°. 

 The faint blue which the sky exhibits at the earth's surface is 

 therefore not the blue of the first order, and is merely the blue 

 of the second or third order rendered paler by the light reflected 

 from the aqueous vapour in the lower regions of the atmo- 

 sphere. 



Immediately after the values of D, I have placed the values of 

 R, or the degree of maximum polarization, in order to show the 

 relation between these two quantities ; but as the values of D 

 were taken only when it was convenient, the numbers R do not 

 show the maximum intensity of the polarization of the atmosphere. 

 I have therefore selected the following from several hundreds of 

 observations recorded in my journal. 







Rotation 



Rotation 





Mean time. 

 h m 



4 30 



in zenith. 



in horizon. 



1841, May 12. 



30l 







2b\ 



12. 



7 10 



30! 



28! 



12. 



7 35 



30| 



29! 



1842, April 13. 



7 32 



32| 





16. 



7 37 



32 



29 



Sept 29. 



4 37 



30| 





Very frequently 



the value of R was 



29 





The following observations show the changes which take place 

 in the maximum polarization in a few hours : — 



* Newton's * Optics/ 3rd edit. Book II. part 3. prop. vii. p. 232, See 

 also prop. v. p. 228, from which it would appear that by " small parcels " 

 Newton meant solid globules of water. 



