324 On the Affections of Light 



penetrate the topaz at R, and after reaching the posterior sur- 

 face ai, it will be partly transmitted at C in the direction CF, 

 and partly reflected in the direction Cr, so as to depart from 

 the point C almost wholly polarized by reflection ; but in its 

 passage from C to r along the oblique depolarizing axis of the 

 crystal, it is depolarized and emerges at r, in the direction rr' 

 deprived of the polarity which it had acquired by reflection at C. 

 If the observer now looks into the topaz in the direction /r, 

 through a plate of agate having its laminae perpendicular to the 

 plane of the section ABab, he will perceive about ten brilliantly 

 coloured elliptical rings, four of which, with the two central 

 spots, are shown in Plate VI. fig. 1.* 



The following measures will convey a correct idea of their 

 form and magnitude. 



Breadth of the central spots including half the black 



space between them 1° iiV 



Distance of the outsides of the central spots 3 42 



Transverse length of each central spot 5 7 



Extreme conjugate diameter oi first red ring 7 24 



Ditto second 11 6 



Ditto third 14 48 



Ditto fourth 18 30 



Ditto fifth 22 12 



Ditto sixth 25 54 



Ditto seventh 29 36 



Ditto eighth 33 18 



Ditto ninth 37 



Ditto tenth 40 42 



Black space between the oval centres 144- 



Til order to convev a correct notion of the diiferent colours 

 which compose the elliptical rings, and which vary in different 

 parts of the same ring, I have given in Plate VII. f, fig. 1, an 

 outline of the first six rings with references to the following 

 table, which contains the colours in five diflferent parts of the 

 semi-circumference of each ring. 



f 1. /./f/i^ W/(e with a purplish tinge . fS. JB/ari fading into light blue to- 

 fading into lihile above, and ^ ( vrards 6". 



gradually deepening into black i •^ 6. Light Hue fading into green. 



below. . j 7. yi?//ou' shading into re<i. 



2 J 2. /f'A!(>' fading into yellow above, " (^S. Deep crimson. 



and %/i< Wi(e below. , g g^^,^ very little 

 Ydlow shaduig off into white fe ^^ ^,,; ^^ x\n\e, the gree« 



below, and red above. "H i beginning a little below? 



^ , " ^.^'^;.^".'^ ^ P'"!^ ""g^' an'J ° I 11. yw/o«. shading into red. 



V. shading into j/e«oK,' below. 3 [_ 12. Crimson. 



* I have counted fourteen of these rinps when the fi_<;ht was polarized by 

 oblique transniissioii through a plate of mica 0127th of an inch tkick. 

 The colours are in this case nuich more distinct. 



t This Phite will appear in our Hcxt Number. 



