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XIX. 



A NEW FOEM OF POLARIMETER FOR THE MEASUREMENT 

 OF THE REFRACTIVE INDEX OF OPAQUE BODIES. 



By W. F. BARRETT, F.R.S., 

 Professor of Experimental Physics, Royal College of Science for Ireland. 



[Eead April 20. Ordered for Publication May 11. Published July 27, 1909.] 



In 1814 Sir David Brewster, after an extensive series of measurements of the 

 angle of maximum polarization of various reflecting surfaces, was led to the 

 well-known and important law which bears his name, viz., that the index of 

 refraction of any substance is the tangent of the angle of maximum 

 polarization for that substance. Hence, when a ray of light incident on a 

 transparent body is polarized by reflection, the reflected ray forms a right 

 angle with the refracted ray. 



By means of Brewster's Law the indices of refraction of various opaque 

 non-metallic reflecting surfaces have been obtained. As every different 

 colour has a different index of refraction, the law shows that the polarizing 

 angle correspondingly varies with the different rays of the spectrum, being, 

 for a given substance, smallest in the red and largest in the violet. In bodies 

 of low dispersive power the angle of maximum polarization is nearly the 

 same for all colours, and white light can be used as the source. In other 

 cases monochromatic light must be employed — either a sodium flame or 

 suitably coloured glass in front of the source described below. 



The amount of light reflected from some opaque bodies is small ; and hence 

 the determination of the polarizing angle is difficult, unless we can always 

 keep the analyser placed in the reflected beam at the same angle as the ray 

 incident on the opaque surface under examination. To secure this I have 

 devised the following instrument (fig. 1), whereby with a rack- work and simple 

 link-motion, the collimator, which renders the incident rays parallel, and the 

 telescope, in which is placed the analysing Nicol's prism, are simultaneously 

 moved througli equal angles. The opaque body is placed on a movable 

 table with levelling screws, which are adjusted until the reflecting surface is 

 level, and at the centre of the graduated circle, round which travel the 

 telescope and collimator. A small but brilliant source of light is employed, 

 such as a Nernst or a 10-volt electric glow-lamp; a small lens throws a 

 brilliant image of the light on to the adjustable slit of the collimator. This 



