AND TRANSMITTED BY CERTAIN KINDS OF GLASS. 
283 
Table XII., and also tliose for crown glass from Table XIX., are given in the second 
and third columns of Table XX. The amount of light which, according to Fresnel’s 
theory, should have been reflected by the glass was calculated for the various 
incidences by means of the formula 
T 3 _ 1 [ sin^ (i — r) tan- (i — r) ] ^ 
’’ ^ \sin® {i + r) taii^ {i + r)J 
the values of r being determined from the observed value of the refracting index of 
the glass ; the fourth column contains the results. 
By assuming the truth of the theory, the value of the refractive index could, of 
course, be readily deduced from the amount of light reflected at a nearly normal 
incidence, this being equal to 
- 1\3 
n 1 
Before repolishing, the crown glass reflected 
3’78 per cent, of the incident light; hence, the value of n would be 1’4842. Assuming 
that such was the case, the amount of light reflected by the glass at various angles 
was calculated, and these numbers are given in the fifth column. 
Table XX. 
Percentage amount of Light Beflected by Crown Glass. 
Angle of incidence. 
Observed. 
Calculated. 
Before 
repolishiug. 
After 
repolisliing. 
Prom observed 
value of n. 
From calculated 
value of n. 
O 
6 
17 -{ 
6 
47 > 
3-78 
4-29 
4-19 
7 
30 J 
10 
0 
3-80 
4-41 
4-19 
3-81 
20 
0 
3-77 
4-81 
4-21 
3-81 
30 
0 
3-92 
4-55 
4-34 
3-93 
40 
0 
4-52 
5-30 
4-77 
4-34 
50 
0 
5-53 
6-27 
5-98 
5-52 
56 
34 
^ 7-22 
8-24 
7-62 
7-23 
60 
0 
8-54 
9-88 
9-16 
8-63 
65 
0 
11T6 
12-28 
12-31 
11-75 
70 
0 
15-49 
18-28 
17-37 
16-78 
75 
0 
26-33 
25-58 
The percentage amount of light reflected before and after repolishing and the 
amount calculated from the observed index of the glass are represented by the carves 
on Plate 8, fig. 4, where the abscissae are the angles of incidence, and the ordinates 
the percentages. The curve for the values deduced from the index calculated from the 
amount of light reflected normally is not given, as it coincides so closely with the 
curve for the glass in its original state that, in order to render the differences visible, 
it would have been necessary to draw the diagram on a much larger scale. 
2 o 2 
