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the centre, and in the plane, of a large divided circle, 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 Fresnel'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 

 formulae which he has proposed for the case of metallic re- 



