97 
measuring the comparative Intensities of Light. 
arrive after having been reflected, are found to illuminate 
equally the field of the photometer, will be an accurate mea- 
sure of the loss of the light in reflection. 
The following table will shew the results of five experi- 
ments with a small, but most excellent glass mirror, made 
by Ramsden. This mirror, which makes part of an opti- 
cal instrument I caused to be constructed in London about 
twelve years ago, is 7 inches long, and 5^ inches wide, and I 
suppose is as perfect as ever glass mirror was of that size. 
To facilitate the comparison of the results of the experi- 
ments, the lamp B, at the beginning of each experiment 
(when the intensity of its direct rays was compared with the 
intensity of the standard lamp), was placed at the distance of 
100 inches, the standard lamp being occasionally moved, in 
order to produce an equality of the shadows. 
Experi- 
ments. 
The an- 
g!e of 
dence. 
Distance of the 
centre of the mir- 
ror from the cen- 
tre of the field. 
Distance of the 
lamp from thecen- 
tre of the mirror. 
Real distance of 
the lamp, or 
length of the re- 
flected rays. 
Light lost in the 
reflection. 
inches. 
inches. 
inches. 
parts. 
1 
6 o° 
40 
40,8 
80,8 
,3472 
2 
8,5° 
— 
4L 
8 l, 
>3439 
3 
45° 
— 
4 L5 
8l,5 
>3338 
4 
6o° 
— 
39,5 
79>5 
,3680 
5 
70° 

4 0,5 
80,5 
>3520 
The mean of these five experiments gives for the loss of 
light ,3494 ; and from hence it appears, that more than ^ part 
of the light which falls upon the best glass mirror that can be 
constructed is lost in reflection. 
The loss with mirrors of indifferent quality, is still more con- 
siderable. With a very bad common looking-glass the loss, 
mdccxciv. O 
