2o6 Dr. Brewster on the Affections of Light 
If the plate of agate is now turned round 90®, so that its 
laminas are parallel to the plane of the section ABah, 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 complementary to those in the first set. By measuring the 
diameters of the red rings in the second set, it will be found 
that they correspond with those of the gree 7 i rings in the first 
set; the blue rings correspond with the yellow ; the green with 
the red; and the yellow with the blue ; and in the outer rings 
the blue with the pink, and the pink with the blue. The central 
spots in the second set exhibit the same opposition of colours 
to those in the^r^^ set; but they are smaller, and placed at a 
greater distance ; and the space around them which was for- 
merly black is now white. 
If instead of a plate of agate we employ a doubly refracting 
crystal, the first set of rings will, in one position of the crystal, 
be seen in the first image; and upon turning the crystal about 
its axis, the first set will occupy the second image, and the 
second set the first image, an alternation taking place 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 used, as the nebulous image formed by this mineral 
injures, in some degree, the distinctness of the image; but on 
the other hand, the doubly refracting crystal requires to be cut 
into a prism with a large angle, in order to separate the two 
images which it forms, and therefore it alters the shape of the 
rings, and produces a complete change upon their colours.* 
• Since this paper was written, I have discovered a new property of light in virtue 
