INTERFERENCE OF POLARIZED LIGHT. 211 



conceive to how many phenomena these repeated cross- 

 ings of light may give rise ; and how superfluous it 

 would have been to seek the reason of them as long as 

 the laws of interference were unknown. Let us only 

 remark that nothing as yet has indicated whether these 

 laws be equally applicable when, before the rays mix, 

 they have received that modification of which I have 

 already spoken, and which is designated by the name of 

 polarization. 



This question was important ; it formed the object of 

 a difficult investigation, which Fresnel undertook in con- 

 junction with one of his friends (Arago). The example 

 which they have set in publishing their researches, of 

 distinguishing which portion each of them contributed, if 

 not with respect to the material execution of the differ- 

 ent experiments, at least to the invention of them, de- 

 serves, I think, to be followed. For associations of this 

 kind often produce mischief, because the public persist- 

 ing, often through blind caprice, in not treating the par- 

 ties concerned on a footing of perfect equality, may 

 improperly excite the self-love of an author ; perhaps of 

 all human passions that which requires the most control. 



Let us look at the results of the researches in ques- 

 tion, as, without reference to the important consequences 

 which have been deduced from them, they deserve to be 

 stated, were it only on account of their intrinsic singu- 

 larity. 



Two rays which are made to change directly from the 

 state of common light to that of rays polarized in the 

 same direction, preserve, after having received that modi- 

 fication, the property of interfering as before ; they rein- 

 force or destroy each other as ordinary rays do, and 

 under the same conditions. Two rays which change 



