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 
