OF ARTS AND SCIENCES. 



say that, in the former, the transmitted rays are computed by the formula 

 , and in the latter, where the internal reflection is neg- 



1 — a 



1 + (m — 1) a 



lected, by the formula (1— «) 



TABLE VII. 



Polarization, supposing there is no internal reflection. 



Part II. Experimental. 



To test the above conclusions, two experimental methods may be 

 employed. First, by means of a photometer, to determine the amount 

 of light in any given case ; and, secondly, by means of a polarimeter, to 

 determine the percentage of polarization of the reflected and refracted 

 rays. The latter method has been employed in the following experi- 

 ments. The instrument commonly used to measure the amount of 

 polarization was invented by Arago, and is called a polarimeter. It 

 consists of a Nicol's prism and Savart's plates, in front of which are 

 several glass plates, free to turn, and carrying an index which moves 

 over a graduated circle, thus showing the angle through which they 

 have been rotated. The prism and plates form a Savart's polariscope, 

 which gives colored bands with either light or dark centre, according 

 as the plane of tlie prism is parallel or pei'pendicular to the plane of 

 polarization. When the plates are so placed that the light passes 

 through them normally, they have no effect on it ; but when turned, 

 they polarize it in a plane parallel to the axis of rotation, and by an 

 amount dependent on the angle. Let the instrument be so set that the 

 axis of rotation shall be perpendicular to the plane of polarization, and 

 the plates set at zero. The bands will then be visible, the centre one 

 being bright. As the plates are turned, the bands become fainter, until 

 their polarization neutralizes that originally present in the beam ; beyond 

 this point the bands reappear dark-centred. The amount of polariza- 



VOL. I. 2 



