yo Count Rumford’s Account of a Method of 
responding to one of the lights in question being deeper or 
fainter, according as it is more or less enlightened by the other, 
when the shadows are equal, the intensities of the illuminating 
rays must be equal likewise. 
In removing the lights, in order to bring the shadows to be 
of the same density, care must be taken to recede from, or ad- 
vance towards the centre of the paper in a straight line, so that 
the one light may always be found exactly in the line of re- 
flection of the other ; otherwise the rays from the different 
lights falling upon the paper, and consequently upon the sha- 
dows, at different angles, will render the experiment fallacious. 
When the intensity of one strong light is compared with the 
intensities of several smaller lights taken together, the smaller 
lights should be placed in a line perpendicular to a line drawn 
to the centre of the paper, and as near to each other as pos- 
sible ; and it is likewise necessary to place them at a greater 
distance from the paper than when only single lights are com- 
pared. 
In all cases, it is absolutely necessary to take the greatest 
care that the lights compared be properly trimmed, and that 
they burn clear, and equally, otherwise the results of the ex- 
periments will be extremely irregular and inconclusive. It is 
astonishing what a difference there is in the quantities of light 
emitted by the same candle, when it burns with its greatest 
brilliancy, and when it has grown dim for want of snuffing. 
But as this diminution of light is progressive, and as the eye 
insensibly conforms to the quantity of light actually. present, it 
is not always taken notice of by the spectators ; — it is never- 
theless very considerable, in fact, as will be apparent to any 
one who will take the trouble to make the experiment ; and 
