Astronomical and Nautical Collections. 187 



Having placed the two halves of a plate of sulfate of lime, 

 about one twentieth of an inch in thickness, before two 

 slits cut in a screen, and turning these plates in such a man- 

 ner that their axes were perpendicular to each other, I 

 examined, by means of a rhomboid of carbonate of lime, the 

 direction of the polarisation of each of the two groups of 

 fringes which they produced. We have seen that the right 

 hand group results necessarily, according to the known laws 

 of interference, from the combination of the extraordinary 

 rays of the right hand plate with the ordinary rays of the 

 left, since these latter move the more rapidly in the sulfate of 

 lime : this group must therefore be polarised perpendicu- 

 larly to the principal section of the right hand plate, since 

 this is the direction of the polarisation both of the ordinary 

 rays of the left, and of the extraordinary on the right, ac- 

 cording to the actual arrangement of the plates : and since, 

 besides, direct experiments on the interference of rays po- 

 larised in any plane, show always that the fringes are 

 polarised in the same plane. In the same manner, the group 

 on the left, resulting from the interference of the ordinary 

 rays on the right with the extraordinary rays on the left, 

 will be polarised perpendicularly to the principal section of 

 the plate on the left. Now these consequences of our hypo- 

 thesis are perfectly conformable to experiment : for we find 

 when the principal section of the rhomboid, placed before 

 the lens, is parallel to the axis of the right hand plates, the 

 ordinary image contains no other than the left hand fringes, 

 and the extraordinary image the right hand fringes; and on 

 the contrary, when the principal section of the rhomboid is 

 parallel to the axis of the left hand plate, or perpendicular to 

 that of the right hand plate, it is the left hand group that 

 has disappeared from the ordinary image, and the left hand 

 from the extraordinary. 



We see that the ordinary and extraordinary rays are here 

 distinguished, not by their direction, as when the crystal is 

 cut into the form of a prism, but by the difference of their 

 effects of interference. Thus, for example, in the space 

 occupied by the fringes of the right hand group, which re- 

 sult from the interference of the extraordinary rays of th^ 



