ON LIGHT. 381 



fering in phase by half an undulation, and which there- 

 fore (by what we have before shown) compound a single 

 ray polarized in a plane half-way intermediate, or 45 

 inclined to the original plane of polarization ; whereas a 

 ray of ordinary light so transmitted would show no signs 

 of polarization in any one plane more than in any other. 

 (159.) The most remarkable cases of circular polar- 

 ization, however, are those which occur when a ray is 

 transmitted along the optic axis of a crystal of quartz, 

 and some few other crystals, as also through certain 

 liquids. The phenomena so exhibited cannot be ex- 

 plained, or even described, however, till we shall have 

 said something 



OF THE COLOURS EXHIBITED BY CRYSTALLIZED PLATES 

 ON EXPOSURE TO POLARIZED LIGHT. 



(160.) Uniaxal crystals. If a plate cut from a crystal 

 of Iceland spar, so as to have its faces perpendicular to 

 the axis of the primitive rhomboid, be placed close to 

 or very near the eye ; and before it a tourmaline plate 

 having its axis vertical, so as to polarize all the light in- 

 cident upon it in vertical planes passing through the 

 eye ; and if any brightly illuminated white surface, such 

 as a white cloud, or a sheet of paper laid in the sun- 

 shine, be viewed through it : or if, instead of a tourma- 

 line plate, a "polarizing frame" of glass plates, such as 

 above described, be laid horizontally, and the reflexion 

 of a clouded sky be in like manner viewed through the 

 crystal; in the "polarized field" so obtained nothing 



