Professor Airy on the Double Refraction of Quart:. 327 



are described as they appear when examined with an analyzing 

 plate of unsilvered glass. If a plate of tourmaline be used, the 

 right and left parts of the image will have the same relative po- 

 sition, but the upper and lower will be interchanged : the ob- 

 server's eye being supposed to turn in such a manner that the 

 axis of the tourmaline appears up and dozen. 



Phenomena. — I. If a plate of calc spar cut perpendicular to 

 the axis be examined with the polarizing and analyzing plates 

 crossed, the uniaxal system of rings is produced. If the ana- 

 lyzing plate be turned less than 90° either way round the inci- 

 dent ray, the complementary system is produced. The order 

 of colours does not sensibly differ from Newton's scale, begin- 

 ning t. ith black. These are common and well known pheno- 

 mena. 



II. If FresneFs rhomb of glass be placed to receive the po- 

 larized light, so that the plane of reflexion pass through the divi- 

 sions 45° and 225°, the calc spar will present another appearance. 

 The rings are abruptly and absolutely dislocated : those in the 

 upper right hand quadrant and the quadrant opposite to it are 

 pushed from the center by one-fourth of an interval, and those 

 in the other quadrants are drawn nearer to the center by the 

 same quantity. The line separating the quadrants is nowhere 

 black : the intensity of its light is uniform and almost equal to 

 the mean intensity. If the plane of incidence pass through 

 135° and 315°, the phenomena of adjacent quadrants are exact- 

 ly interchanged. No alteration is made by turning the analy- 

 zing plate round the incident ray : the lines dividing the qua- 

 drants are always parallel and perpendicular to the plane of re- 

 flexion at the analyzing plate. (See Note E.) 



III. If the plane of reflexion in the rhornb pass through 0° 

 and 180°, or through 90 J and 3(i0°, the phenomena are precise- 

 ly the same, and undergo the same changes as those in Pheno- 

 menon I. If, while the plates are crossed, the rhomb be turned 

 gradually from the position 0° towards 45°, the rings are gra- 

 dually changed, at first becoming (as far as the eye can judge) 

 elliptical, and then assuming the form represented in Fig 8. 



IV r . If a plate of quartz, whether right or left-handed, he in- 

 terposed between the crossed plates, a set of rings is seen. As 

 far as the eye can judge, the rings are exactly circular, hut there 



