measuring the comparative Intensities of Light. Sg 
and without any visible smoke, emitted only about ~ part as 
much light as the Argand's lamp ; these lamps being placed 
over against each other before the field of the photometer, 
their lights were found to be in equilibrium when the less 
being placed at the distance of 20 inches from the centre 
of that field, the greater was removed to the distance of 101 
inches. I now concluded, that if the less light were to be 
removed to the distance of 40 inches, it would be necessary, in 
order to restore the equilibrium of light, or equality of the 
shadows in the field of the photometer, to remove the greater 
light to the distance of 202 inches ; that is to say, if the dimi- 
nution of the light arising from the imperfect transparency of 
the air should not be perceptible within the limits of that dis- 
tance. But if, on the contrary, it should be found upon re- 
peated trials, that the equilibrium was restored when the 
greater light had arrived at a distance short of 202 inches, I 
might thence conclude, that such effect might safely be attri- 
buted to the imperfect transparency of the air : for notwith- 
standing that the light of the smaller lamp would of course be 
diminished as well as that of the greater, yet as there is every 
reason to suppose that the diminution, whatever it may be, 
must ever be proportional to the distance through which the 
light passes in the medium, as the augmentation of the dis- 
tance through which the light of the smaller lamp passes is no 
more than 20 inches, while that of the greater is made to pass 
through an additional distance, amounting to more than 100 
inches, it is evident that the diminution of the light of the 
greater lamp, arising from the imperfect transparency of the 
medium, must be greater than the diminution of the light of 
the less lamp, arising from the same cause ; and, consequently, 
mdccxciv. N 
