of the Light emitted by Luminous Bodies. 23 



render that resistance sensible within the confined 

 limits of my machinery. 



Having provided two lamps, the one an Argand's 

 lamp, which I made to burn with the greatest possible 

 brilliancy; the other a small common lamp, with a 

 single, round, .and very small wick, which, burning 

 with a very clear, steady flame, and without any visi- 

 ble smoke, emitted only about 2T P ai "t 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 

 smaller 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 

 smaller 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 

 diminution of the light arising from the imperfect 

 transparency of the air should not be perceptible 

 within the limits of that distance. But if, on the con- 

 trary, it should be found upon repeated 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 at- 

 tributed to the imperfect transparency of the air; for 

 notwithstanding 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 



