68 Count Rumford's Account of a Method of 
Let the two burning candles, lamps, or other lights to be 
compared, A and B, be placed at equal heights upon two light 
tables, or moveable stands, in a darkened room ; let a sheet of 
clean white paper be equally spread out, and fastened upon the 
wainscot or side of the room, at the same height from the 
floor with the lights, and let the lights be placed over against 
this sheet of paper, at the distance of 6 or 8 feet from it, and 6 
or 8 feet from each other, in such a manner, that a line drawn 
from the centre of the paper, perpendicular to its surface, shall 
bisect the angle formed by lines drawn from the lights to that 
centre ; in which case, considering the sheet of paper as a 
plane speculum, the one light will be precisely in the line of 
reflection of the other. 
This may be easily performed, by actually placing a piece of 
a looking-glass, 6 or 8 inches square, flat upon the paper, in 
the middle of it, and observing by means of it the real lines of 
reflection of the lights from that plane, removing it afterwards 
as soon as the lights are properly arranged. 
When this is done, a small cylinder of wood, about f of an 
inch in diameter, and 6 inches long, must be held in a vertical 
position about 2 or 3 inches before the centre of the sheet of 
paper, and in such a manner, that the two shadows of the cy- 
linder corresponding to the two lights may be distinctly seen 
upon the paper. 
If these shadows should be found to be of unequal densities, 
which will almost always be the case, then that light whose 
corresponding shadow is the densest, must be removed farther 
off, or the other must be brought nearer to the paper, till the 
densities of the shadows appear to be exactly equal ; or in other 
words, till the densities of the rays from the two lights are 
