the Light reflected by the Sky and by Plates of Glass. 131 



another Table was computed by the formula (l — a) m , or sup- 

 posing that no internal reflection took place. A comparison 

 showed that while the reflected beam is affected but little, a 

 great change takes place in the transmitted light. The results 

 are shown by the dotted lines in figs. 7, 8, and 10, and will be 

 discussed below. 



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 experiments. The instrument com- 

 monly used to measure the amount of polarization was invented 

 by Arago, and is called a polarimeter. It consists of a NicoFs 

 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 pola- 

 riscope, which gives coloured bands with either light or dark 

 centre, according as the plane of the prism is parallel or perpen- 

 dicular 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 pa- 

 rallel to the axis of rotation, and by an amount dependent on 

 the angle. Let the instrument be so set that the axis of rota- 

 tion shall be perpendiular 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 the polarization neutralizes that originally present 

 in the beam ; beyond this point the bands reappear dark-centred. 

 The amount of polarization is thus readily determined by turn- 

 ing the plates until the bands disappear, when the angle is re- 

 duced to percentages by means of a Table. The difficulty of 

 computing this Table, however, is the real objection to the use 

 of this instrument. It may be determined by the formula? given 

 in the first part of this paper ; but it, of course, then fails to 

 prove them. Moreover no account is taken of imperfect trans- 

 parency, dust on the surface, and other sources of error. An 

 excellent way of forming this Table experimentally is to view 

 through the instrument a beam of light totally polarized. If 

 now the plane of polarization of the beam is changed, the per- 

 centage of polarization will alter, being zero when it is inclined 

 45° to the axis of the plates, and wholly polarized at an angle of 

 0° or 90°. At any angle a the beam may be regarded as com- 

 posed of two, eos 2 a polarized vertically, and sin 2 a polarized 



K2 



