Astronomical and Nautical Collections. 189 



nately to the right and left of the principal section, sometimes 

 arriving at the primitive plane, and sometimes at another 

 plane situated at the same angular distance on the other side, 

 or at the azimuth 2i, calling the angle formed by the two 

 first planes i. For example, if the principal section makes 

 an angle of 45°, with the primitive plane of polarisation, the 

 axes of the particles vibrate through an arc of 90°, which is 

 now 2t. Mr. Biot supposes that these oscillations are repeated 

 a very great number of times before the particles attain a 

 fixed polarisation, which arranges their axes, so as to make 

 them either parallel or perpendicular to the principal sec- 

 tion : and a thickness of some tenths of an inch, or perhaps 

 of some inches, is required, according to this able experi- 

 menter, in order that the moveable polarisation should 

 become fixed in the sulfate of lime, at least while the paral- 

 lelism of the two surfaces prevents the separation of the 

 ordinary and extraordinary pencils, which is always accom- 

 panied by the fixed polarisation. But when the faces are 

 parallel, and the thickness of the plate does not exceed the 

 limit, the particles of light which pass through it, instead of 

 being polarised in the principal section, and in the direction 

 perpendicular to it, are polarised either in the primitive 

 plane, or at the azimuth 2z, accordingly as the last oscillation 

 of their axes was directed towards the first or the second 

 plane, and this whether it was finished or only begun at the 

 time of their emersion; at least, according to Mr. Biot, 

 they are affected by the rhomboid which is employed for 

 analysing the emergent light, as if their last oscillation had 

 been finished. The time occupied by one of these oscillations, 

 or the thickness of the crystal in which each of them is per- 

 formed, is supposed to be constant for particles of the same 

 nature, but variable in the different kinds of light, in pro- 

 portion to the length of the ** fits" [imagined by Newton.] 

 Let us now examine the consequences of this, and consider 

 the case of the two halves of a plate of sulfate of lime, about 

 the tenth of an inch in diameter, placed before two mirrors 

 of black glass in the path of the reflected rays. Let us sup- 

 pose that the mirrors, disposed in such a manner as to pro- 

 duce the fringes, are inclined in an angle of 35° to the rays 

 which proceed from the luminous point, so that they may b^ 



