" This is a very mysterious and a very beautiful phenomenon when 

 observed by the aid of a polariscope, consisting of a tourmaline plate, 

 with a slice of Iceland crystal or nitre, cut at right angles to the optic 

 axis, and applied on the side of the tourmaline farthest from the eye. In 

 a cloudless day, if the sky be explored in all parts by looking through 

 this compound plate, the polarized rings will be seen developed with more 

 or less intensity in every region but that nearest the sun and that most 

 distant from it the maximum of polarization taking place on a zone of 

 the sky 90 from the sun, or in a great circle, having the sun for one of 

 its poles, so that the cause of polarization is evidently a reflection of the 

 sun's light on something. The question is, on what ? Were the angle of 

 maximum polarization 76 we should look to water or ice for the reflect- 

 ing body. But though we were once of this opinion (art. Light, Encycl. 

 Metropol. 1143), careful observation has satisfied us that 90, or there- 

 abouts, is the correct angle, and that therefore, whatever be the body on 

 which the light has been reflected, if polarized by a single reflection, the 

 c polarizing angle ' must be 45, and the index of refraction, which is the 

 tangent of that angle, unity ; in other words, the reflection would require 

 to be made in air upon air ! The only imaginable way in which this could 

 happen would be at the plane of contact of two portions of air differently 

 heated, such as might be supposed to occur at almost every point of the 

 atmosphere in a bright sunny day ; but against this there seems to be an 

 insuperable objection. The polarization is most regular and complete, as 

 we have lately been able to satisfy ourselves under the most favorable 

 possible atmospheric conditions, after sunset, in the bright twilight of a 

 summer night, with the sun some degrees below the horizon, and long 

 after all the tremor and turmoil of the air, due to irregular heating, must 

 have completely subsided. On the other hand, if effected by several suc- 

 cessive reflections, what is to secure a large majority of them being in 

 one plane (in which case only their polarizing effect would accumulate) ; 

 and of those which become ultimately effective, what, is there to deter- 

 mine an ultimate deviation of 90 as that of the maximum ? The more 

 the subject is considered, the more it will be found beset with difficulties ; 

 and its explanation, when arrived at, will probably be found to carry 

 with it that of the blue color of the sky itself." 



SIR JOHN HERSCHEL. 



