-transmitted through crystallized Bodies. 327 



from the first surface of the topaz ; but the second set of rings 

 is very faint, as the light RS is not extinguished by the polarizing 

 bodv. 



When we examine the transmitted light CF, either with the 

 naked eye or with polarizing crystals, no coloured fringes are 

 visible. 



Such are the modifications which common light undergoes in 

 its passage through topaz. The affections of polarized light, 

 which now come under consideration, are still more varied and 

 interesting. In my first experiments on this subject, I po- 

 larized the light by transmitting it through the agate ; but 1 

 afterwards found it most convenient to communicate this pro- 

 perty by reflection from the surface of a transparent body. 



Let RR', Plate IV., fig. 9, be a beam of polarized light ob- 

 tained by reflection from any transparent body GH, the plane 

 of reflection from GH being 'perpenthcular to the plane of re- 

 flection from the topaz AB. A part of this beam will be re- 

 flected at C in the direction Cr, and part of it transmitted at 

 C in the direction CF, no light being reflected from the first 

 surface AB. Tlie ravs transmitted at C having been polarizecl 

 before their incidence at R' are depolarized in passing from R' 

 to C along the ol)lique depolarizing axis, and the rays reflected 

 at C are polarized by reflection from the surface ab, and again 

 depolarized in their passage from C to r along the other oblique 

 depolarizing axis. ^ 



If the observer now looks into the topaz in the direction rr , 

 he will perceive the first set of elliptical coloured rings, as re- 

 presentetl in Plate VI., fig. 1. These rings are now peculiarly 

 distinct and brilHant, and it was therefore from them that I drew 

 up the table of colours referred to from Plate VII., fig. I. 



Let the rav rr be now received upon a plate of agate having 

 its lamina; perpendicular to the section ABe^', and a third set 

 of rings will be seen like those in Plate VI., fig. 3. This third 

 set differs from t!ie frst set only in the central parts. All the 

 Tings have the same colours in'both ; but the central spots are 

 much smaller in the third set than in the first, and the mass of 

 darkness wif.h which they are surrounded encroaches considerably 

 upon the blue part of the first ring. 



In the third set of rings the distance of the outsides of 



the two central spots is '' '^ 



Conjugate diameter of each spot • • • * ' 



Ditto of the black space between the 



1 n 

 spots » " 



The third set, indeed, may be considered as the exact coun- 

 terpart of the second set, all the colours of the former being 



X 4 «ora- 



