188 FORMATION OF IMAGES. SECT. XXII. 



polarized in different planes. In all intermediate posi- 

 tions, fringes of intermediate brightness are produced. 

 The analogy of a stretched cord will show how this 

 happens. Suppose the cord to be moved backward and 

 forward horizontally at equal intervals ; it will be thrown 

 into an undulating curve lying all in one plane. If to 

 this motion there be superadded another similar and 

 equal, commencing exactly half an undulation later than 

 the first, it is evident that the direct motion every mole- 

 cule will assume, in consequence of the first system of 

 waves, will at every instant be exactly neutralized by 

 the retrograde motion it would take in virtue of the 

 second ; and the cord itself will be quiescent in conse- 

 quence of the interference. But if the second system 

 of waves be in a plane perpendicular to the first, the 

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

 ference would take place. Rays polarized at right an- 

 gles to each other may subsequently be brought into the 

 same plane without acquiring the property of producing 

 colored fringes ; but if they belong to a pencil the whole 

 of which was originally polarized in the same plane, they 

 will interfere. 



The manner in which the colored 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 colored phenomena. If the 

 analysis be made with Iceland spar, the single ray pass- 

 ing through the optic axis of the mica will be refracted 

 into two rays polarized in different planes, and no in- 

 terference will happen. But when two rays are trans- 

 mitted 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 colors of the other image, when the 



