STRUCTURE AND LIGHT OF THE SKY. 265 



showed the green at its maximum. At a further distance 

 of 45 from the position of maximum red, the color disap- 

 pears a second time. We have there a second neutral 

 point, beyond which the green comes again into view, at- 

 taining its maximum brilliancy at the end of a rotation of 

 180. By the rotation of the Nicol, therefore, through an 

 angle of 90, we produce a color complementary to that 

 with which we started. 



As may be inferred from this result, the selenite ring- 

 system changes its character when the Nicol is turned. 

 It is possible to have the centre of the circle dark, the 

 surrounding rings being vividly colored. The turning of 

 the Nicol through an angle of 90 renders the centre bright, 

 while every point occupied by a certain color in the first 

 instance is occupied by the complement of that color in the 

 second. By curious internal actions, not here to be de- 

 scribed, the cloud in our experimental tube sometimes 

 divides itself into sections of different textures. Some sec- 

 tions are coarser than others, while it often happens that 

 some are iridescent to the naked eye, and others not. 

 Looking normally at such a cloud through the selenite and 

 Nicol, it often happens that in passing from section to sec- 

 tion the whole character of the ring-system is changed. 

 You start with a section producing a dark centre and a 

 corresponding system of rings ; you pass through a neutral 

 point to another section and find there the centre bright, 

 and each of the first rings displaced by one of the comple- 

 mentary color. Sometimes as many as four such rever- 

 sions occur in the cloud of an experimental tube a yard 

 long. Now, the changes here indicated mean that in passing 

 from section to section of the cloud the plane of vibration 

 of the polarized light turns suddenly through an angle of 

 90 ; this change being entirely due to the different texture 

 of the two parts of the cloud. 



You will now be able to understand, as far as it is cat 

 12 



