96 
Count Rumford’s Account of a Method of 
Of the Loss of Light in its Reflection from the Surface of a plane 
Glass Mirror. 
In these experiments the method of proceeding was much 
the same as in those just mentioned. The lamps A and B 
burning with clear, bright, and steady flames, were placed be- 
fore the field of the photometer, and one of them was moved 
backwards and forwards till the illuminations of the shadows 
in the field of the instrument were found to be precisely equal. 
The distance of the lamp B being then noted, this lamp was 
removed, and a mirror being put in its place, but nearer the 
field of the photometer, the lamp was so placed that its rays* 
striking the centre of the mirror, were reflected against the 
field of the photometer, where, by bringing the lamp nearer 
to, or removing it farther from the mirror, the illumination of 
the field by those reflected rays was now brought to be in 
equilibrium with the illumination of the standard lamp, and 
then the distance of the lamp from the centre of the mirror, 
and the distance from thence to the centre of the field, were 
carefully measured, and noted. These two distances added 
together, was the real distance through which the rays passed 
in order to arrive at the field of the photometer. 
Now as there is always a loss of light in reflection, it is evi- 
dent that the reflected rays must come to the field of the pho- 
tometer weakened, and that in order to illuminate this field 
by these reflected rays as strongly as it was illuminated by the 
direct rays of the same lamp, the lamp must be brought nearer 
to the field. It is likewise evident, from what has already 
been said, that the ratio of the squares of those distances of 
the lamp when its rays pass on directly, and when they 
