154 Dr. Brewster on the action of 
eye , the pencil is polarised i8o° out of the plane of reflection, 
or it has again returned into that plane. 
In passing through the last 45° of azimuth, the polarisation 
varies very slowly, the change being only about io°; whereas 
in passing through the first 42° of azimuth, the polarisation 
varies no less than 90°, indicating in the most unequivocal 
manner, as we shall afterwards see, that this change depends 
upon the angle which the incident ray forms with the axis of 
the crystal. 
The light reflected from the separating surface of the oil 
and the spar is a maximum , when the plane of the principal 
section is perpendicular to the plane of reflection, and its 
colour is then nearly white. When these two planes coincide, 
the intensity of the light is a minimum , and its colour is then 
a faint red ; and in intermediate positions, the reflected pencil 
has both its intensity and its colour of an intermediate 
character. In the azimuth of 42°, the reflected pencil exhibits 
a very curious phenomenon when analysed with calcareous 
spar. Its colour is then yellowish white, and all the yellow 
light is polarised transversely to the plane of reflection. One 
of the images, however, instead of vanishing, consists of blue 
and red light, the red vanishing, and the blue becoming more 
brilliant as the analysing prism is turned to the left ; and the 
blue vanishing, and the red becoming more brilliant as the 
prism is turned to the right. This effect arises from the diffe- 
rence in the angles at which the red and blue rays are incident 
upon the separating surface of the oil and the spar. Each 
set of rays, therefore, as will afterwards appear, suffers a 
different change of polarisation, the one being polarised about 
87® out of the plane of reflection, and the other 93°. 
