48 FRBSXEI^ ON THE COLOURS PKOUUCEU IN 



occurs, Avhen a tube, however long, filled with oil of turpentine 

 is placed between the two glass parallelepipeds. Thus the mo- 

 difications imparted to the incident rays are not altered in this 

 case by the interposition of the fluid. 



When, instead of placing the glass parallelepiped at the fore- 

 most extremity of the tube, it is placed at the end nearest the 

 eye, the polarized light, which, after traversing the oil, is re- 

 flected twice in this parallelepiped, presents the characters of 

 a pencil of light which has traversed a thin lamina parallel to 

 the axis ; for, on turning the rhomboid of calcareous spar, the 

 nature of the tints is no longer varied, but only their intensity, 

 which pass into a perfect white in two rectangular positions of 

 its principal section, when it is inclined 45^ to the plane of double 

 reflexion. The tints arrive, on the contrary, at their greatest 

 intensity when the principal section of the rhomboid is parallel 

 or perpendicular to this plane. Their nature depends upon the 

 position of the glass parallelepiped, and is precisely that of the 

 colours obtained directly without its interposition, when the 

 principal section of the rhomboid of calcareous spar is brought 

 to the same azimuth. 



In thus modifying by double total reflexion the polarized light 

 which has traversed oil of turpentine, the effects of this liquid 

 may be combined with those of a crystallized lamina cut parallel 

 to the axis, in the same manner as the effects produced by two 

 such laminae are combined. But in order that the addition or 

 subtraction of the tints may be effected in a perfectly similar 

 manner, to obtain, for instance, the total disappearance of one 

 of the images with a lamina of suitable thickness, it is necessary 

 that the plane of double reflexion should be turned in a certain 

 azimuth depending upon the length of the tube ; this azimuth, 

 in the particular case of perfect compensation, is that which gives 

 the same tint as the crystallized lamina. When the axis of the 

 lamina is to the left of the plane of double reflexion, the tints are 

 added ; when \i is to the right, they are subtracted. This order 

 would be inverted with a fluid like oil of citron, in which the 

 polarizing action is in a contrary direction to that of oil of 

 turpentine. 



In the last memoir which I had the honour of presenting to 

 the Academy, I described an apparatus, by means of which, with 

 a crystallized lamina cut parallel to the axis, the phaenomena of 

 colorization produced by oil of turpentine and plates of rock- 



