Astronomical and Nautical Collections. 391 



If we polarise by reflection at the surface of a plate of 

 glass, blackened at its second surface, the diverging rays 

 proceeding from a luminous point, and then cause them to 

 pass through two rhomboids of equal thickness, placed one 

 before the other, and having their principal sections perpen- 

 dicular to each other, and at the same time inclined in an 

 angle of 45° to the plane of reflection, it is known that the 

 two pencils, produced by this pair of rhomboids, can only ex- 

 hibit fringes when they are brought back to common planes 

 of polarisation, by the assistance of a third rhomboid, or a 

 pile of glass placed before or behind the lens. The most 

 advantageous direction of the principal section of the third 

 rhomboid is that which makes an angle of 45° with the prin- 

 cipal sections of the two others; because then each of the 

 two pencils which are emitted by these is divided equally 

 into the ordinary and extraordinary images produced by the 

 third rhomboid ; and this equality of the two systems of un- 

 dulations, which interfere in each image, renders the points 

 of complete discordance as dark as possible : and if the light 

 were perfectly homogeneous, they would in this case be com- 

 pletely black. The apparatus being thus arranged, if we 

 consider any one point in the group of fringes, that, for ex- 

 ample, which occupies the middle, and answers to equal paths 

 described by the two constituent pencils of each image, it 

 may be remarked that there is a maximum of light in the 

 ordinary image, when the principal section of the rhomboid 

 is parallel to the primitive plane of polarisation, which we 

 may call horizontal, and that the same point, on the contrary, 

 is black in the extraordinary image, that is, the light is re- 

 duced to 0. It is restored by degrees as the rhomboid is 

 turned, and its intensity augments in proportion as the prin- 

 cipal section is removed from the horizontal direction:, when 

 it is inclined in an angle of 45°, the light of this point is as 

 great in the ordinary as in the extraordinary image : at last 

 it disappears entirely in the ordinary image, and becomes a 

 maximum in the extraordinary, when the principal section is 

 vertical. We see then that the whole light, collected in this 

 point, presents all the characters of a complete polarisation 

 in a horizontal plane. If we now consider the point which 

 answers to the difl'erence of a quarter of an undulation in the 



