XII. ON A NEW OPTICAL INSTRUMENT, INTENDED 

 CHIEFLY FOR THE PURPOSE OF MAKING EXPERI- 

 MENTS ON THE LIGHT REFLECTED FROM METALS. 



[Proceedings of the Royal Irish Academy, April 9, 1838.] 



THE instrument consists of two hollow arms or tubes, moveable 

 about 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 interposed 

 between the eye-piece and the centre of the circle. At this centre 

 is placed a stage for carrying the reflector, with its plane perpen- 

 dicular to the plane of the circle, and having 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 rota- 

 tion. When the two arms are set at equal angles with the re- 

 flector, 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 

 position in which the light will entirely disappear. An obser- 

 vation will then have been made ; for the light, before its inci- 

 dence 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 polarized ; 

 and our object is to determine the position and species of the 



