Polarization of the Light reflected by the Sky. 107 
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 center, according as the plane of the prism is paralle 
or perpendicular to the plane of polarization. When the plates 
are so placed that the light passes through them normally, they 
have uo effect on it; but A turned, they polarize it in a 
al parallel to the axis of rotation, and by an amount depen- 
ent 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 
perfect transparency, dust on the surface, and other sources of 
error. An excellent way of forming this table experimentally 
1s to view through the instrument a beam of light totally polar- 
wed. If now the plane of polarization of the beam is changed, 
the percentage 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 re- 
arded as composed of two, cos*a polarized vertical y, sin?a 
polarized horizontally. The percentage of polarization will 
therefore equal tate ranThas 2a, from which the polariza- 
tion corres ins Gee en angle is readily determined. 
ndi any given 
The feat of scl a Sites is given in Table IIL Four 
Series of observations were taken, from which curves were con- 
structed as in fig. 8, with angles of incidence as abscissas and 
percentages of polarization as ordinates. A curve was next 
wn, coinciding with them as nearly as possible, and its ordi- 
nates are given in Table ILI, column 3; the angles of incidence 
are given in the first column, and the theoretical polarization 
in the second column of the same table. Column 4 gives the 
differences, and from it we see that, while the agreementis very 
close between 0° and 60°, above this point a marked variation 
18 perceptible. This deviation will be further discussed in con- 
nection with fig. 8. 
