Astronomical and Nautical Collections. 181 



the inferences, which have been drawn from the first. I 

 took a rhomboid of calcarious spar, polished on two oppo- 

 site faces, which were carefully made parallel, and sawed it 

 perpendicularly to these two faces, so that I had two rhom- 

 boids exactly equal in thickness, and in which the paths of 

 the rays were consequently of equal lengths at equal incli- 

 nations. I placed them one before the other, so that the rays 

 which passed perpendicularly through the one passed in a 

 similar manner through the other : the principal section of 

 the one was also perpendicular to that of the other, so that 

 there were only two pencils which pervaded them, the ordi- 

 nary pencil of the first being extraordinarily refracted in the 

 other, and the reverse. Now it resulted from this arrange- 

 ment, that the differences of the paths depending on the dif- 

 ferent velocities of the ordinary and extraordinary rays, were 

 compensated in each of the pencils. The pencils crossed each 

 other in a very small angle, so that the fringes which they 

 would have formed must have had a much greater breadth 

 than was sufficient for their being visible ; and yet, notwith- 

 standing that all the conditions necessary for the production 

 of fringes, in common cases, were strictly observed, I could 

 never succeed in obtaining them. While I looked carefully 

 for them, with a lens in my hand, I caused the direction of 

 one of the rhomboids to vary slowly, moving it sometimes to 

 the right, and sometimes to the left, in order to compensate 

 for the effect of any difference in the thickness, if it existed : 

 but although I repeated this trial a number of times, I still 

 observed no fringes ; and indeed they were not to be ex- 

 pected, considering the experiments which have been related, 

 since the two pencils emerging from the rhomboids were po- 

 larised at right angles. 



It was not for want of a correct adjustment that the ex- 

 periment did not succeed ; since I easily obtained the fringes, if 

 I employed light which had been polarised before its entrance 

 into the rhomboids, when the polarisation is again changed 

 after its emersion. It is therefore completely demonstrated, 

 by the experiments which I have just related, that rays po- 

 larised at right angles cannot exercise any sensible influence 

 on each other ; or in other words, that their coiribination 



