AND TRANSMITTED BY CERTAIN KINDS OE GLASS. 
273 
Table XII.— continued. 
Before rcpolisliing. 
After repolishing. 
Angle of 
incidence. 
Per cent. 
Per cent. 
(a;-a?,). 
{x - X.,) 
of light 
Xi. 
(X-Xi). 
^2* 
{x - 
of light 
reflected. 
reflected. 
70° A. 
169-2 
87-6 
109-1 
147-7 
14-02 
158-7 
92-2 
105-8 
145-1 
17-94 
167-3 
89-5 
109-0 
147-8 
15-57 
157-7 
93-2 
105-2 
145-7 
18-21 
166-0 
90-8 
106-8 
150-0 
15-17 
157-0 
93-9 
104-4 
146-5 
18-10 
105-3 
91-5 
107-4 
149-4 
15-83 
157-0 
93-9 
105-5 
145-4 
18-83 
Mean 
18-28 
B. 
156-9 
94-2 
99-3 
151-8 
15-42 
Mean 
15-49 
75° B. 
160-1 
90-8 
119-4 
131-5 
26-52 
' 159-5 
91-4 
118-9 
132-0 
26-64 
158-7 
92-2 
116-7 
134-2 
25-52 
157-8 
93-1 
117-3 
133-6 
26-83 
1 
Mean 
26-33 
Section V.—Amount of Light Transmitted after Reg)olishing. 
The effect of repolishing on the amount of light transmitted by the glass was 
examined. The first observations, those with the 24’3 mm. plate of crown glass (except 
the last) were made with the photometer used in the reflection experiments, the arm 
of the goniometer carrying the lamp being clamped in the prolongation of the line 
joining the flxed lamp and the axis of the goniometer. The small slot in front of the 
flame of the fixed lamp was subsecpiently replaced by one of the same size as that in 
front of the lamp carried by the goniometer, and most of the observations made with 
this arrangement, in which there were two lamps of nearly equal illuminating pjower. 
Finally, a photometer with a^ single lamp, two mirrors, and inclined paper surfaces, 
like that used for the original transmission experiments, was fitted up, and this was 
used for all measurements made in August and September, 1888, t]:at is, those with 
the flint glass and the four last determinations of the light transmitted by the 
crown glass. 
The measurements were made as described in Section I., but only four readings 
were taken in each position of the glass, and the results were calculated out by the 
. , X. (r — x^) 
expression k = - - - 
X^ (x — X]) 
(see p. 250), where k is the coefficient of transparency, x the 
distance between the two lights, and and x.^, the two positions of the photometer 
m which there is equality of illumination, the optical shortening of the path of the 
light due to its passage through the glass being of course allowed for. 
The results are contained in Tables XIII. and XIV. 
MDCCCLXXXIX.—A. 2 N 
