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the centre, and in the plane, of a large divided cirele, each arm 
being provided with a Nicol’s eye-piece, or some equivalent 
contrivance for polarizing light in a single plane ; while in one 
arm, which is of course crooked, a F'resnel’s rhomb is interpos- 
ed between the eye-piece and the centre of the circle. At 
this centre is placed a stage for carrying the reflector, with 
its plane perpendicular to the plane of the circle, and hav- 
ing a motion to and fro for adjustment. Each eye-piece, 
as well as the Fresnel’s rhomb, turns freely about the axis 
of the arm to which it belongs, and is provided with a 
small circle for measuring its angle of rotation. When the 
two arms are set at equal angles with the reflector, and the 
observer looks. through the crooked arm, he will see a light 
admitted through the straight one; and then, by turning the 
Fresnel’s rhomb, and the eye-piece next his eye, he will be 
able, by means of their combined movements, to find a po- 
sition in which the light will entirely disappear. An obser- 
vation will then have been made; for the light, before its in- 
cidence on the metal, is polarized in a given plane by the first 
eye-piece ; but after reflexion from the metal, (as we know 
from Sir David Brewster’s experiments,) it is elliptically po- 
larized; and our object is to determine the position and 
species of the little ellipse in which the reflected vibration is 
supposed to be performed. Now, the axes of this ellipse are 
parallel and perpendicular to the principal plane of the 
rhomb, when it is in the situation above described, where 
the light completely disappears; and the ratio of the axes is 
the tangent of the angle which that plane makes with the 
principal section of the eye-piece next the eye. The angles 
are read off from the divided circles; and thus, for any 
angle of incidence, and any plane of primitive polarization, 
we can at once ascertain the nature of the reflected elliptic 
vibration. Professor Mac Cullagh mentioned, that the in- 
strument was made last year with the view of testing certain 
formulee which he has proposed for the case of metallic re- 
