198 Sir D. Brewster on the Phicnomcna of Thin Plates. 



Angles of incidence. Azimntlis. Complements. 



75 

 76 

 80 

 85 

 90 



If we now conceive AB, fig. 3, to be the section of tlie 

 plane of incidence, having the tliff'erent incidences marked 

 upon it from 90° to 5b ll', and if round a centre in AB pro- 

 longed, where 0° of incidence falls, we describe tlieazinmthal 

 circle ZAZ, then the complements of the azimuths of the 

 polarized light being set off from the corresponding angles of 

 incidence on each side of AB, the curves ACB, ACB passing 

 through these points will show at what angles of incidence 

 and azimuth the rings disappear, in consequence of the planes 

 of polarization of the two pencils being at these places rect- 

 angular 



At all incidences, and in all azimuths within the shaded 

 space ACBC, the 'whitc-centred rings are seen, and at all other 

 azimuths and incidences the black-centred rings are seen. 



2. Fluor Spar and Water. — 1 have taken this combination 

 as a specimen of the phasnomena which take place at some 

 incidences less than 90^, when the refracted ray falls on the 

 second surface of the film, at angles greater than its polarizing 

 angle. The following Table shows the values of x and their 

 complements: — 



By projecting these values, as is done in fig, 4, we obtain a 

 double set of curves which unite at D, where the angle of in- 

 cidence is 78" 4', at which the refracteil ray falls upon the 

 second surface at its polarizing angle. 



