POLARIZED LIGHT AT THE SURFACE OF A UNIAXAL CRYSTAL. 605 
Each recorded value of is the mean of five observations. At high angles of 
incidence the difference between two observations of the same value of <f> was some¬ 
times as great as 10'. The error produced in the value of 9 by an error in (f> is, for 
these values of <£, less than S^/2, so that the extreme difference between the values of 
0 calculated from each of the five values of <£, of which the mean is given in the table, 
may be as great as 5'. At high angles of incidence the mean error in the value of 0, 
as given in the table, is considerably less than 2'. 
At lower angles of incidence, 45° to 30°, the differences in the observed values of <f> 
were much less, and rarely amounted to 5', but then the error produced in 9 by an 
error in cf) is not far short of S<£, so that the probable accuracy of the values of 9 
given in the table remains much the same as before. 
Table I. 
0. 
0'. 
0”-0'. 
O 1 1/ 
O / li 
O | u 
83 11 0 
36 41 10 
1 2 40 
78 10 30 
36 4 50 
1 0 50 
68 11 30 
33 57 40 
56 20 
65 34 35 
33 13 10 
54 40 
56 49 0 
30 14 10 
50 20 
52 28 40 
28 30 10 
48 0 
48 32 30 
26 48 10 
46 10 
38 17 50 
21 53 40 
41 10 
31 5 15 
18 5 50 
37 20 
30 36 0 
17 50 10 
37 0 
24 42 0 
14 33 40 
33 0 
21 3 10 
12 29 0 
30 0 
17 45 0 
10 34 10 
26 50 
