262 
SIR J. CONROY ON THE AMOUNT OF LIGHT REFLECTED 
2 4'3 mm. plate. 
Eeadings. 
Percentage of 
light transmitted. 
x'\ 
X*. 
cm. 
cm, 
78-0 
87-0 
89-65 
74-1 
85-9 
86-26 
Mean 87-95 
Ill order to have obtained really accurate results with this method it would have 
been necessary to have made a large number of observations, and taken their mean ; 
but, as the results obtained with it, and with the polarising photometer, agreed fairly 
well with one another and with the far larger number of observations made with the 
first method, it was thought unnecessary to continue the observations, the agreement 
between those obtained by all three methods being sufficient to show that there was 
but slight, if any, probability of a constant error due to the photometer itself used in 
the first series. 
Table VI. gives the results of the three methods and the probable error of each 
determination calculated by the ordinary formula, 0'674 
Table VI.—Percentage Amount of Light Transmitted. 
First method. 
Second method. 
Third method. 
Crown glass — 
6-5 mm. plate. 
91-50 + -06 
91-40 + 1-07 
92-03 + -18 
II-5 „ . 
90-07 „ -07 
90-53 „ -32 
15-0 „ . 
89-13 „ -07 
89-54 „ -22 
18-5 ,, . 
88-51 „ -08 
24-3 „ . 
87-16 „ -15 
, , 
87-95 „ -81 
Flint glass— 
7-0 mm. tlii( k. 
88-83 „ -09 
49-0 „ ..... 
85-40 „ -07 
69-5 „ . 
82-57 „ -11 
91-3 „ . 
80-74 „ -07 
Common plate glass— 
6 mm. thick. 
87-68 „ -10 
3 plates of 6 mm. crown glass . 
88-08 „ -05 
