ON LIGHT. 367 



lipticity of the spheroid in question is determined solely, 

 or even principally, by the degree of obtuseness of the 

 rhomboidal form of the crystal. It appears to be regu- 

 lated far more by the chemical and other physical quali- 

 ties of the material. 



(146.) Of the interference of polarized rays. The assimi- 

 lation of a ray of light to a series of equidistant waves 

 running along a stretched string, will afford a very clear 

 conception of the interference of polarized rays. Sup- 

 pose a vibratory movement in a horizontal plane to be 

 communicated to one end of such a string, and to propa- 

 gate along it such a series of waves, which will therefore 

 all be confined to the same horizontal plane. If then a 

 simultaneous movement, exactly equal and similar, and 

 in the same plane, were communicated to a point in the 

 string exactly half a wave breadth in advance of the point 

 where the first series originated ; each point in its length 

 anywhere in advance of both these origins of movement 

 would be always solicited by two equal and opposite im- 

 pulses, the one of which would contradict the other, and 

 in consequence it would remain at rest, and the two 

 series of waves would destroy one another. If the origin 

 of the two vibratory movements were distant from each 

 other by a whole wave breadth, they would conspire to 

 produce a double extent of vibratory excursion all along 

 the string. All this is merely recapitulatory of what was 

 stated, in Lecture VII., when explaining the general 

 nature of the interference of rays. But it is evident that 

 these conclusions only follow if the interfering vibratory 

 movements are performed in the same plane. Supposing 



