depolarisation of light, &c. 41 
vanished image reappears by the slightest motion of the prism 
of calcareous spar. In some plates of oil of mace, this dark 
interval is occupied with a third image, so that the depolarised 
image has the appearance of being composed of three images 
closely pressing upon each other. In other plates the bright 
image is partly depolarised, even when the luminous sectors 
are visible. 
The phenomena of the luminous sectors will be understood 
from PI. V. fig. 1, where AB is the, plane in which the light, 
of the candle is polarised, and M N the two images of it 
formed by a prism of calcareous spar, m being the place of 
the vanished image, and n the visible image. The evanescent 
image at m is surrounded with the four luminous sectors 1, 3, 
5, 7, separated by dark sectors 2, 4, 6 , 8. The bright image 
of the candle at N is also surrounded with four luminous 
sectors 9, 11,13, 15, separated by dark sectors 10, 12, 14, 16; 
but these sectors, of which 11 and 15 are the brightest, are 
not nearly so luminous as those at M. 
If the oil of mace is kept in one position, while the prism 
of calcareous spar is turned round so as to make the image N 
move about M, as a centre in the direction BC, the evanescent 
image of the candle begins to appear at m : the luminous 
sectors turn round in the direction 1, 2, 3, 4, 5, 6, 7, 8 : the 
sectors 1, 5 grow fainter, and 9, 15 brighter; and after the 
image N has moved through an arch of 45 0 , the two images 
M and N have nearly the same appearance. When the image 
N has described an arch of 90°, the sectors have the appear- 
ance represented in fig. 2, the candle having regained its full 
lustre in the middle of M, and having vanished in the middle 
of n. The sectors 1, 5 are now the faintest of those round 
Mpcecxv. G 
