292 Sir David Brewster on the Polarization 



proaches the meridian, the value of the maximum polarization, 

 diminishes, that this value increases, on the contrary, in a con- 

 tinuous manner as the sun recedes from the meridian, and that 

 it reaches its maximum when the sun is very near the horizon, 

 the amplitude of this variation being about 0*09." 



On the 16th of October, 1854, the maximum polarization in- 

 creased gradually after middav from 25° to 0° of the sun's alti- 

 tude, from 0-6236 to 0*7051 ;" and on the 19th of October, from 

 5° to 35° of the sun's altitude, it diminished from 0*7083 to 

 0*6106. On these two days the maximum polarization, at an 

 altitude of 20°, was 0-6582 and 0-6464 respectively, the mean of 

 which is 0'6523, differing only 0*12 from 0'64, as computed from 

 Fresnel's formula by M. Bernard, from my observation in 1842, 

 that, when the sun's altitude was 20°, the intensity of the maxi- 

 mum polarization at 90° from the sun was equivalent to that 

 which would be produced by reflexion from the surface of glass 

 whose index of refraction was 1*486, at an angle of 65° 30'*. 



Before he became acquainted with the memoir of M. Bernard, 

 Dr. Rubenson had completed his observations on the same sub- 

 ject ; and though they lead to a similar result, yet they possess 

 a peculiar value from their having been made with the finest in- 

 struments, in different localities, at almost all the seasons of the 

 year, and under various states of the atmosphere. 



From a careful examination of his observations, Dr. Rubenson 

 arrives at the general conclusion " that the atmospheric polariza- 

 tion is subject to a diminution during the morning, and to an in- 

 crease during the evening, without one's being able to assign with 

 certainty the precise hour of the minimum polarization." These 

 changes Dr. Rubenson found to be often influenced by pertur- 

 bations, commonly of short duration, and taking place indiffer- 

 ently at all hours of the day. They frequently arise from clouds 

 or smoke, and probably often from cirrus too faint to be seen. 

 According to Dr. Rubenson, the blue colour of the sky, in a 

 normal state of the atmosphere, and 90° from the sun, is feeble 

 at sunrise, increases rapidly in intensity, and attains to its maxi- 

 mum some hours before noon, the number of hours being differ- 

 ent at different seasons. The intensity of the colour diminishes 

 towards noon. It then increases, reaches a second maximum 

 after some hours, and then diminishes quickly towards sunset. 

 The relation between the blue colour of the sky and the inten- 

 sity of its polarization is a problem which remains to be solved. 



In 1859 M. Liais made observations on the polarization of 

 the atmosphere during his voyage from France to Brazil, and 

 at San Domingo in the bay of Rio Janeiro. His observations 



* Johnston's 'Physical Atlas/ "Meteorology/' p. 10; or Phil. Mag. 

 S. 3. vol. xxiv. p. 453, December 184 7» 



