218 INTERFERENCE OF POLARIZED LIGHT. 



nation goes, the phenomena of interference and of colour 

 should be produced bj the crystalline plate alone, and in 

 common light, without either polarizing plate or analyzing 

 plate. Such, however, is not the fact, and the real difficulty 

 in this case is, not so much to explain how the phenomena 

 are produced, as to show why they are not always produced. 



In seeking for a solution of this difficulty, we perceive 

 that the two rays, whose interference is supposed to produce 

 the observed results, are not precisely in the condition of 

 those whose interference we have hitherto examined. They 

 are polarized, and polarized in opposite planes. We are led 

 then to inquire, whether there is anything peculiar to the in- 

 terference of polarized rays which may influence these results ; 

 and the answer to this inquiry will be found to complete the 

 solution of the problem. 



(229) The subject of the interference of polarized light 

 was examined, with reference to this question, by Fresnel and 

 Arago, and its laws experimentally developed. It was found 

 that two rays of light, polarized in the same plane, interfere 

 and produce fringes, under the same circumstances as two 

 rays of common light ; that when the planes of polarization 

 of the two rays are inclined to each other, the interference is 

 diminished, and the fringes decrease in intensity ; and that, 

 finally, when the angle between these planes is a right angle, 

 no fringes whatever are produced, and the rays no longer in- 

 terfere at all. These facts may be established by taking a 

 plate of tourmaline which has been carefully worked to a 

 uniform thickness, cutting it in two, and placing one-half in 

 the path of each of the interfering rays. It will be then 

 found that the intensity of the fringes depends on the rela- 

 tive position of the axes of the two tourmalines. When these 

 axes are parallel, the fringes are best defined ; they decrease 

 in intensity, when the axes of the tourmalines are inclined to 



