Astronomical and Nautical Collections. 395 



Mr. BiOT, guided by the theory of emanation, could not 

 suspect that the light which was polarised in a certain plane, 

 could be composed of rays polarised in different directions, 

 and naturally judged of the direction of the polarisa- 

 tion of the ordinary and extraordinary rays transmitted by 

 the crystallized plate, from that of the whole light. Hence 

 he was led to conclude, that the rays did not undergo in 

 crystallized plates the same kind of polarisation as in crystals 

 thick enough to divide the light into two separate pencils. 

 But this is not a necessary consequence of the phenomenon ; 

 since the experiment with the two rhomboids demonstrates 

 that the same appearances are produced by the combination 

 of two distinct pencils, polarised in directions parallel and 

 perpendicular to the principal section of the crystal: and 

 besides, the hypothesis would be contradictory to other facts, 

 since we have always found the ordinary and extraordinary 

 rays actually polarised in directions parallel and perpendi- 

 cular to the principal sections of the crystals that transmit 

 them. It is, therefore, not to the separate pencils of ordi- 

 nary or extraordinary rays that we must apply what Mr. 

 Biot has said of the mode of polarisation of the light which 

 has passed through a crystal, but to the light collectively: 

 and it is still necessary to modify the proposition of this dis- 

 tinguished observer, in order to render it perfectly correct ; 

 for his words imply that each kind of homogeneous rays is 

 always completely polarised either in the primitive plane, or 

 at the azimuth 2 i. Now we have seen, in the experiment 

 with the two rhomboids, that this complete polarisation is 

 only acquired in particular cases ; and the direct experiment 

 on crystallized plates leads us to the same result. 



In short, all the phenomena exhibited by crystallized plates 

 are easily explained, and even foretold, by the ordinary rules 

 of the laws of interference ; and the small number of parti- 

 cular modifications of these laws, as they relate to the 

 mutual influence of polarised rays, determined by experi- 

 ment. 



Rays which are polarised at right angles to each other 

 have no mutual influence; and it is for this reason that the 

 two systems of undulations transmitted by crystallized bodies 

 do not immediately exhibit any effects of this kind, even 



