328 On the Jffeclions of Light 



complementary to those of the latter, and the central spots hav- 

 ing the same form and magnitude. 



If the plate of agate is now turned round, so that its laminae 

 are parallel to the section ABa/', a fourth set of rings will be 

 seen. This new set, which is represented in Plate VI., fig. 4, is 

 by no means brilliant, but it is distinguished from all the rest by 

 striking peculiarities. In its general structure it resembles the 

 Jirst !>et; but in the middle of each central spot there is a darker 

 spot composed of blue and red, with a little green above the 

 blue, and every ring is divided into two rings, each of which has 

 the same colours as the original ring. This division of the rings 

 occupies only a part of the semi-circumference of each, and is 

 not seen beyond the third ring. 



When the agate begins to move from the position which gives 

 the third set of rings, into that v\hich gives the fourth set, two 

 blue spots and the divisions of the rings begin to appear at 

 a, a, a, a, a', a', a', a', Plate VII. fig. 2, and move along the 

 lines ale, a'i'c till they arrive at c, c, c, c, c , c', c', c'. when the 

 rings assume tiie appearance of the Jour I h set. If the agate 

 performs another revolution of 90" from the position which gives 

 X\\e fourth set into that which gives the third set, the blue spots 

 and the divisions of the rings move off in the direction c, d, e, 

 c', d'. t', till the rings assume the appearance of the third set. 



The phaenomena which have now been described may also be 

 perceived, when the polarization of the rays r/ is effected either 

 by a doubly refracting crystal or by reflection. In one position 

 of the doubly refracting crystal the third set of rings is seen in 

 the first image, and the fourth set in the second image, and they 

 alternate in every quadrant of the motion of the crystal. When 

 the ray r/ is reflected from a transparent body, so that the 

 plane of reflection is parallel to the plane of reflection from the 

 topaz, the fourth set of rings will be visible. 



Hitherto we have attended only to the light reflected from ah, 

 the posterior surface of the topaz ; but the light transn)itted at ' 

 C exhibits also interesting phaenomena. When the observer 

 looks through the topaz in the direction FC, so as to see the 

 polarizing surface GH, the sec07id set of rings is faintly visible. 

 They become extremely distinct, however, when viewed through 

 a plate of agate having its laminae at right angles to the plane 

 ABab. If the lamina are parallel to the plane ABab, the second 

 set is converted into i\\e first set with colours a little paler than 

 when it was produced in the former experiments. 



In the preceding experiments the plane of reflection from GH 

 has been perpendicular to the plane of reflection from the topaz. 

 We shall now describe the phgeuomena which take place when 



these 



