transmitted through crystallixed Bodies, 209 
complementary to those of the latter, and the central spots 
having the same form and magnitude. 
If the plate of agate is now turned round, so that its laminae 
are parallel to the section ABj6, 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 re- 
sembles the first set, 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^ings occupies only a part of the semicircum- 
ference 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 which gives x\\q fourth set, 
two blue spots and tlie divisions of the rings begin to appear 
at a, a, a, a, a* , a*, a!, a', Plate VII., fig. 3, and move along the 
lines ahe, a'b'c' till they arrive at c, c, c, c, c', d, d , U , when the 
rings assume the appearance of xho. fourth set. If the agate per- 
•1. 
forms another revolution of 90° from the position which gives 
ihefoicrth 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, 
d, d', d till the rings assume the appearance of the third set. 
The phenomena which have now been described may also 
be perceived, when the polarisation of the rays rd 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 rd is reflected from a transparent 
E e 
MDCCCXIV. 
