ON THE POLARIZATION OP LIGHT. 243 



continuing the turn, two other faint images will appear ; fur- 

 ther on, four images of equal brightness, and at 180" they will 

 all coalesce into one bright image, c. 



If a film of selenite be placed between the two crystals, we 

 shall see three coloured images instead of two colourless ones, 

 fig. 156, a'; of the three, the two outer may be green, the 

 middle red. By turning the crystal nearest the eye, the 

 middle image will gradually divide until the completion of the 

 quarter of a revolution, when four images wiU appear, as at b', 

 two being red, the other two green; revolve the crystal 

 another quarter of a circle, the three images, as at d, will 

 re-appear, but with different properties — the two outer being 

 now red, and the middle green; at another quarter four 

 images will be seen, also with their colours reversed, as at d', 

 and at the completion of the circle the original appearance 

 will again occur. 



Wlienever polarized light passes through certain crys- 

 tals in the direction of the optic axis, a series of beau- 

 tifully coloured rings will be seen. Certain angles of 

 any given crystal must be ground down to a plane sur- 

 face and be polished, in order to exhibit the rings; when 

 this is accomplished, there will be found in some positions 

 of the analyzing prism a black cross, with a series of 

 rings around it, and in others a white cross, with the 

 colours of the rings reversed. In crystals having two axes 

 of double refraction, a double system of rings will be seen. 

 Nitre is a beautiful instance of this kind, and a transverse 

 section of a prism of this substance, when ground down and 

 polished, will, with polarized light, exhibit the double system 

 of rings ; but if the prism be ground in some other directions, 

 colour will be produced ; hence it becomes necessary, in order 

 to exhibit the phenomena of colour, to have crystals cut either 

 in the direction of the axis of double refraction, or in a plane 

 inclined at certain angles to it. But when the same sub- 

 stances, in a state of solution, are crystallized on glass, it 

 frequently happens that many of the crystals will be arranged 

 with their axes of double refraction in the direction of the beam 

 of polarized light ; all such, therefore, will exhibit colours, as 

 16* 



