-316 Professor Airy on a new Analyzer, 



conveys no trace of any plane of polarization : and must not vary 

 as the crystal or the analyzer is turned round. 



In the common exhibition of the coloured rings (the incident 

 light' being plane-polarized., the analyzer being the common ana- 

 lyzing plate, and the inclination of the planes of polarization 

 being any whatever) the principal trace of the planes of polari- 

 zation is in the uncoloured brushes. In uniaxal crystals they 

 form an eight-rayed star, composed of two square crosses inclined 

 at an angle equal to that between the planes of polarization, 

 every ray of which separates complementary rings. In biaxal 

 crystals they compose two pairs of rectangular hyperbolas, the 

 angle between whose asymptotes is the same as that between the 

 planes of polarization, and whose branches divide complementary 

 rings. The two crosses or the two sets of hyperbolas unite when 

 the planes of polarization are parallel or perpendicular. 



The first conclusion then is that, in the case under consider- 

 ation, the rings exhibited by crystals will not be traversed by any 

 brushes. Plates of Iceland spar, if they exhibit any variations of 

 light at all, will exhibit circular rings without a cross: and plates 

 of biaxal crystals will exhibit complete lemniscates without any 

 interruption from curved brushes. 



The next conclusion is that the brightness of the light at 

 the poles of the image will depend only upon this consideration ; 

 whether the direction of the circularly polarized light which is 

 incident on the crystal is the same as the direction of that which 

 the analyzer can transmit to the eye, or is the contrary. If it is 

 the same, since the light which forms the poles of the image is 

 that which is not separated into an ordinary and extraordinary 

 ray, and therefore passes unaltered through the crystal, then the 

 light incident on the analyzer is exactly of the kind which the 



