326 On the jiffectiom of Light 



If the plate of agate is now turned round 90", 8o that its la- 

 minae are parallel to the plane of the section kV>al, a second set 

 of elliptical rings will be seen as represented in Plate VI., fig, 2, 

 which is on the same scale as fig. 1 , and which contains only 

 the four first orders of colours, and the central spots. This new 

 set of rings is composed of colours which are complementanj 

 to those in the first set. By measuring the diameters of the red 

 rings in the second set, it will be found that thev corres]iond with 

 those of the green rings in the Jirst set; the Hue rings corre- 

 spond with the yellow; the gree7i with the red ; and the i/ellouf 

 with the Line ; and in the outer rings the blue with the plnkf 

 and the pink with the bhie. The central spots in the se- 

 cond set exhibit the same opposition of colours to those in the 

 first set ; but they are sninller, and placed at a greater distance j 

 and the space around them which was formerly black is now 

 tuliile. 



If instead of a plate of agate we employ a doublv refracting 

 crystal, the Jirst set of rings will, in one position of the crystal, 

 be seen in Llie first image ; and ur)on turning the crystal a1}out 

 its axis, the first set will occupy the second image, and the 

 second set the first image, an alternation taking ])iace in every 

 quadrant of the motion of the crystal. This ^method of view- 

 ing the rings is in some respects superior to that in which the 

 agate is URcd, as the riCl)uIous image formed by this mineral in-^ 

 jures, in some degree, tlie distiiu'tacss of tiie image ; but on the 

 other hand, the doubly refracting crystal rc((uires to be cut into 

 a prism with a large angle, in order to separate the t'lvo images 

 which it forms ; and therefore it alters the, shape of the rings, 

 and produces a complete change upon their colours*. 



If the emergent rays rr', instead of being transmitted through 

 agate or Iceland spar, are reflected at the polarizing angle from 

 any transparent body having its reflecting surface parallel to the 

 plane of the section ABab, they will exhibit thejirst set of rings j 

 but if the reflecting surface is perpendicular to the plane of the 

 section, the second set of rings will be visible. When thejirst 

 set, thus seen by reflection, is examined through a prism of 

 Iceland spar, it suffers no change either in the first or second 

 image. 



In these experiments thejiist set of rings is extremely distinct, 

 as the polarizing crystal extinguishes the light RS reflected 



* Since lliis paper was written, I Imve discovered a new property of 

 Jight, in virtue of wliicli it is polarized by oblique transmission tliiough 

 transparent bodies. Hence, in all my experiments on the coloured riiiij;s, 

 I find it of inraiculable advantage to polarize the light by bnnillcs of glass" 

 plates, and to use them in every case where I formerly employed agate or 

 fralcareous spar. », 



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