194 FORMATION OF IMAGES. SECT. XXII 



especially from glass at angles very little different from the law 

 of the tangents. 



Water polarizes light circularly when between the points of 

 maximum density and solidification ; hence it becomes crystalline. 



The coloured images from polarized light arise from the inter- 

 ference of the rays (N. 216). MM. Fresnel and Arago found 

 that two rays of polarized light interfere and produce coloured 

 fringes if they be polarized in the same plane, but that they do 

 not interfere when polarized in different planes. In all inter- 

 mediate positions, fringes of intermediate brightness are produced. 

 The analogy of a stretched cord will show how this happens. 

 Suppose the cord to be moved backwards and forwards horizon- 

 tally at equal intervals ; it will be thrown into an undulating 

 curve lying all in one plane. If to this motion there be super- 

 added another similar and equal, commencing exactly half an 

 undulation later than the first, it is evident that the direct motion 

 every molecule 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 fr the second ; and the 

 cord itself will be quiescent in consequence 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 

 interference would take place. Eays 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 

 polarized 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 analyz- 

 ing 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 trans- 

 mitted 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, 



