84 Professor Airy on the 



exactly 90°, it is that of fig. 3. The order of colours does not 

 sensibly differ from Newton's scale, beginning with black. 

 These are common and well known phenomena. 



II. If Fresnel's rhomb of glass, mounted as represented in 

 fig. 4, be placed to receive the polarized light, so that the plane 

 of reflection pass through the divisions 45° and 225°, the calc 

 spar will present the appearance of fig. 5. 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 no-where black: the in- 

 tensity of its light is uniform and about equal to the mean 

 intensity. If the plane of incidence pass through 135" and 315°, 

 the phenomena of adjacent quadrants are exactly interchanged. 

 No alteration is made by turning the analyzing plate round the 

 incident ray : the lines dividing the quadrants are always pa-, 

 rallel and perpendicular to the plane of reflexion at the ana- 

 lyzing plate*. 



III. If the plane of reflexion in the rhomb pass through 

 0° and 180°, or through 90° and 360°, the phenomena are precisely 

 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 gradu- 

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

 elliptical, and then assuming the form represented in fig. 6. 



IV. If a plate of quartz, whether right or left-handed, be 



* It is proper to mention that I had exhibited this phenomenon to the Cambridge Philo- 

 sophical Society in the spring of last year, long before the publication of Dr. Brewster's 

 valuable Memoir in the Phil. Trans, for 1830, and (I believe, but I do not recollect the date) 

 before its communication to the Royal Society. 



