igo Dr. Brewster on the Affections of Light 
room through a narrow recta’ gular aperture. When this 
aperture was viewed through the agate, it was surrounded 
with a very considerable nebulosity; and by interposing a 
prism of Iceland spar between the agate and the eye, and giv- 
ing it a motion of rotation, the nebulous light became very 
dense when the bright image vanished, and almost completely 
disappeared when the bright image had reached its greatest 
brilliancy. The bright and the nebulous images, therefore, 
comported themselves exactly like the two images formed by 
doubly refracting crystals; and the small portion of nebulous 
light, which surrounded the bright image at its maximum 
lustre, was obviously produced either by the imperfect polish 
of the agate, or by its not being cut exactly at right angles to 
the plane of its laminae. 
It will be seen from a subsequent section of this letter, that 
light polarised by the agate, or by any other means, is depo- 
larised, or partly restored to its original state, by being trans- 
mitted in a particular direction through a plate of mica, or 
any other crystallized body. I therefore interposed a plate of 
mica between the agate and the Iceland spar when the nebu- 
lous light had nearly disappeared, and having adjusted it to 
the depolarising position, the nebulous light was instantly 
revived round the bright image, while the other bright image 
which had disappeared resumed its place in the middle of the 
other nebulous mass. 
When a pencil of light polarised and afterwards depolarised, 
in a manner to be afterwards described, is transmitted through 
a plate of agate, the red^' rays go to the formation of the bright 
* The red and the green are complementary to each other. The same result is 
obtained if th.e. blue and yellow, or any other two conipleinentary colours are used. 
