ON THE POLARISATION OF THE ATMOSPHERE. 237 



On the 29th June 1850, at about 8^ 30" p.m., several reddish-white clouds 

 appeared in the south-west sky at different heights, and in the zone of maximum 

 polarisation. They were illuminated by the setting sun, and the sky around, 

 and of course behind them, was of a deep blue. Upon looking at one of these 

 clouds through Nicors prism, I found that its reddish-white light was polarised 

 in a plane at right angles to that in which the light of the sky was polarised. 

 When the sky was dark by the disappearance of the blue polarised light in one 

 position of the prism, the cloud was bright ; but when the sky was brightest in a 

 rectangular position of the prism, the cloud was of a dark blue colour. 



July 15, 1850, 9^ 12°' p.m. — All the clouds to the east of the plane passing 

 through the sun and moon, between the south-west and south, are black seen 

 against the sky ; but when we view them with a Nicol's prism, so as to extinguish 

 as much as possible the polarised light of the sky, the clouds are white seen 

 against the dark sky. When the Nicol's prism is turned round 90°, they again 

 become black. 



July 1, 1850, 8^ 30"° p.m. — A fine rainbow, with the secondary and super- 

 numerary bows, appeared in the south-east. When the bands of the polariscope 

 crossed either of the two bows at right angles, the bands at the intersection were 

 very brilliant. When the rainbows were invisible from the great faintness of 

 their light, they became visible, that is, the invisible portion became visible, when 

 crossed with the bands of the polariscope. This effect did not seem to be pro- 

 duced when the bands crossed the supernumerary bows. 



When the sun shines upon a light transparent vapour interposed between the 

 observer and terrestrial objects, these objects are indistinctly seen through the 

 light reflected by the vapour. As this light is partially polarised, it may be 

 extinguished by a Nicol's prism, or a pile of thin plates of glass, or by reflection 

 at the polarising angle from a glass plate. The terrestrial objects are then seen 

 with great distinctness. This mode of obtaining improved vision of objects 

 imperfectly visible, or of seeing objects not otherwise visible, may occasionally 

 be of great use at sea. 



On the Theory of Atmospherical Polarisation. 



When the atmosphere is illuminated by the sun, his rays fall upon the aerial 

 particles which compose it at all angles of incidence. In the immediate vicinity 

 of the sun, where the angle of incidence is 180", there is no polarisation. The 

 polarisation increases with the angle of incidence, and becomes a maximum, 

 as we have seen, at about 90° from the sun. It now diminishes with the angle 

 of incidence, and becomes nothing at 180°, the point opposite to the sun. 



At all these points the polarisation is said to be vertical, being in the vertical 

 plane passing through the sun and the observer. 



