238 FORMATION OF IMAGES. [SECT. xxn. 



second system of waves be in a plane perpendicular to the 

 first, the effect would only be to twist the rope, so that no 

 interference would take place. Rays polarized at right 

 angles to each other may subsequently be brought into the 

 same plane without acquiring the property of producing 

 coloured fringes; but, if they belong to a pencil the whole 

 of which was originally polarize^ in the same plane, they 

 will interfere. 



The manner in which the coloured images are formed may 

 be conceived, by considering that, when polarized light passes 

 through the optic axis of a doubly refracting substance, as 

 mica, for example, it is divided into two pencils by the 

 analyzing tourmaline ; and, as one ray is absorbed, there can 

 be no interference. But, when polarized light passes through 

 the mica in any other direction, it is separated into two white 

 rays, and these are again divided into four pencils by the 

 tourmaline, which absorbs two of them ; and the other two, 

 being transmitted in the same plane with different velocities, 

 interfere and produce the coloured phenomena. If the 

 analysis be made with Iceland spar, the single ray passing 

 through the optic axis of the mica will be refracted into two 

 rays, polarized in different planes, and no interference will 

 happen. But, when two rays are transmitted by the mica, 

 they will be separated into four by the spar, two of which 

 will interfere to form one image, and the other two, by their 

 interference, will produce the complementary colours of the 

 other image, when the spar has revolved through 90 ; be- 

 cause, in such positions of the spar as produce the coloured 

 images, only two rays are visible at a time, the other two 

 being reflected. When the analysis is accomplished by 

 reflection, if two rays are transmitted by the mica, they 

 are polarized in planes at right angles to each other. 

 And, if the plane of reflection of either of these rays be 

 at right angles to the plane of polarization, only one of 

 them will be reflected, and therefore no interference can 

 take place; but in all other positions of the analyzing 



