176 Sir David Brewster on the Polarization 



On the 29th of June, 1850, at about 8 h 30 m p.m., several red- 

 dish-white clouds appeared in the south-west sky at different 

 heights, and in the zone of maximum polarization. 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 a Nicol's prism, I found that its reddish- 

 white light was polarized in a plane at right angles to that in 

 which the light of the sky was polarized. When the sky was 

 dark by the disappearance of the blue polarized light in one posi- 

 tion 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 h 12 m 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 polarized light of the sky, the clouds are white seen 

 against a dark sky. When the Nicol's prism is turned round 

 90°, they again become black. 



July 1, 1850, 8 h 30 m p.m. — A fine rainbow, with the secon- 

 dary and supernumerary 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 po- 

 lariscope. This effect did not seem to be produced when the 

 bands crossed the supernumerary bows. 



When the sun shines upon a light transparent vapour inter- 

 posed between the observer and terrestrial objects, these objects 

 are indistinctly seen, through the light reflected by the vapour. 

 As this light is partially polarized, it may be extinguished by a 

 Nicol's prism, or a pile of thin plates of glass, or by reflexion at 

 the polarizing 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 Theonj of Atmospherical Polarization. 



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

 upon the aerial particles which compose it at all angles of inci- 

 dence. In the immediate vicinity of the sun, where the angle 

 of incidence is 180°, there is no polarization. The polarization 

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

 as we have seen, at about 90° from the sun. It then diminishes 



