ON THE POLARISATION OF THE ATMOSPHERE. 235 



Omitting the three extreme values of D, — viz., 79° 15', 99°, and 120°, — the mean 

 of all the other values is 89° ; but, considering that five of the values of D are 

 marked as more than 90°, we may conclude that 90° is, in the normal state of 

 the atmosphere, the distance from the sun of the place of maximum polarisation, 

 and 45° the corresponding angle of incidence. 



This determination of the place and angle of maximum polarisation affords a 

 highly probable explanation of the azwe colour of the sky. Sir Isaac Newton 

 considers this colour as a " Blue of the first order, though very faint and little, 



for all vapours, when they begin to condense and coalesce into small 



parcels, become first of that bigness whereby such an azure must be reflected."* 



Professor Clausius considers the vapours to be vesicles or bladders, and 

 ascribes the blue colour of the first order to reflection 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 '• 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 polarising 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, and 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 pro- 

 duced by vesicles of water, but must be caused by reflection from the molecules 

 of air, whose polarising 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 atmosphere. 



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

 degree of maximum polarisation, 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 polarisation of the 

 atmosphere, I have therefore selected the following from several hundreds 

 of observations recorded in my journal. 



,, _,- Rotation in Rotation in 



fllean Time. rz n u • 



Zenith. Horizon. 



1841, April 13. 7^ 32™ 32i° 



„ 16. 7 37 32 29° 



1841, May 14. 4 30 30-i 25 



„ „ 7 10 301 281 



* Newton's Optics. 3d 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. 



VOL. XXIII. PART II. 3 T 



