460 Sir David Brewster on the Polarization 



light different from that of the sky ; the degree of polarization 

 is always affected whenever we measure it in parts of the sky 

 which have luminous clouds or illuminated terrestrial objects 

 in their vicinity, or any luminosity in the field of view of the 

 polarimeter. If the neutral point happens to be above or 

 below any such object, its distaiice from the antisolar point or 

 from the sun is increased or diminished*. 



V. On the Maximum Polarization of the Sky. 



After having ascertained the position of the neutral points, 

 or yoles of no-polarization as we may call them, the next most 

 important element to be determined is the maximum •polariza- 

 tion of the atmosphere. 



When a ray of common light is reflected from any trans- 

 parent body, at an angle whose tangent is equal to the index 

 of refraction, it is completely polarized ; or when a ray of light, 

 completely polarized in a plane inclined 45° to the plane of 

 reflexion, is reflected from any such body, its plane of polari- 

 zation is brought into the plane of reflexion ; that is, its plane 

 is turned routid 45°. Hence complete polarization is measured 

 by a rotation of 45°. When the polarized ray is reflected at 

 angles above or below the angle of maximum polarization, its 

 plane is less turned round, and its rotation is more or less 

 than 45°, according as the angle of reflexion is more or less 

 distant from the angle of maximum or complete polarization f. 



Different degrees of rotation below 45° may also be pro- 

 duced by the refraction of the polarized ray at one or more 

 surfaces of glass if, the rotation increasing with the angle of 

 incidence. Hence we may measure the degree of polarization 

 wherever it exists, by observing at what angle of incidence it 

 is compensated or neutralized, by reflexion from a transparent 

 surface, or by refraction at one or more such surfaces. I have 

 found the last method the most convenient, and have therefore 

 constructed a polarimeter which measures the polarization of 

 the sky, by observing with it either the varying angle at which 



* On the 16th of May 1842, barometer 30*3, the sun was faintly seen 

 through a thick haze. At S*" 49' a.m. the polarization was positive all the way 

 from the sun to the horizon, so that the neutral point below the sun was 

 below the horizon. Immediately afterwards the sun was quite hid — a great 

 glare supervened, and a quaquaversiis polarization was observed, in which 

 the polariscope gave no coloured bands. 



On the 17th of May, at 6" 30', the sun's disc was quite white through a 

 thick haze, and there was no neutral point either above or opposite the sun, 

 the polarization being everywhere positive. When the haze is thicker on 

 one side of the plane passing through the sun's spectrum, the neutral point 

 deviates from that plane. 



f See Phil. Trans., 1830, p. 69. - + Ibid. p. 133. 



