108 EL. C. Pickering— Measurements of the 
TasLeE IIl.—Table for Arago’s Polarimeter. 
<7 Theoretical.| Empirical. | Difference. 
i 
0° 0-0 0-0 0 
20 5-1 5:0 —6.1 
30 11°4 13.0 +1 
40 21°8 23.5 17 
50 oO 37.0 19 
55 42°0° 43.0 10 
60 48°9 49.0 01 
65 54.1 57.5 3:4 
70 56.3 63.5 71°2 
75 55.8 67.0 11°2 
80 52.6 72.0 19°4 
To avoid the defects of the above instrument, the following 
arrangement has been employed. A brass tube, A B, fig. 3, 
percentage of polarization is then readily determined from the 
angle through which the prism has been turned. To determine 
the law which connects these two, let the plane of polarization 
be vertical, and the line of junction of the two images parallel 
to it. Then call A and B the brightness of the two images Te- 
spectively, in which case the polarization p = ae If the 
