366 PHILOSOPHICAL TRANSACTIONS. [aNNO 1794, 



light is propagated in perfectly transparent or unresisting spaces, or where, suf- 

 fering no diminution whatever from the medium, its intensity is weakened 

 merely in consequence of the divergency of the rays ; and as it is more than 

 probable that air, even in its purest state, is far from being perfectly transparent. 

 For greater perspicuity Count R. arranges all the experiments and inquiries under 

 general heads, and begins by prefixing to those which relate to the subject 

 now under consideration, the general title of " Experiments on the Resistance 

 of the Air to Light." 



Exper. 1. Two equal wax-candles, well trimmed, and which were found by a 

 previous experiment to burn with exactly the same degree of brightness, were 

 placed together, on one side, before the photometer, and their united light was 

 counterbalanced by the light of an Argand's lamp, well trimmed, and burning 

 very equally, placed on the other side over against them. The lamp was placed 

 at the distance of 1 00 inches from the field of the photometer, and it was found 

 that the 1 burning candles (which were placed as near together as possible, with- 

 out their flames affecting each other by the currents of air they produced), were 

 just able to counterbalance the light of the lamp at the field of the photometer, 

 when they were placed at the distance of 60.8 inches from that field. One of 

 the candles being now taken away and extinguished, the other was brought 

 nearer to the field of the instrument, till its light was found to be just able, 

 singly, to counterbalance the light of the lamp ; and this was found to happen 

 when it had arrived at the distance of 43.4. inches. In this experiment, as the 

 candles burnt with equal brightness, it is evident that the intensities of their 

 united and single lights were as 2 to 1, and in that proportion ought, according 

 to the assumed theory, the squares of the distances, 60.8 and 43.4, to be ; and 

 in fact, 60.8- = 3696.64 is to 43.4^ = 1883.56 as 2 is to 1 very nearly. In se- 

 veral other experiments the mean of all gave very nearly the same result. 



Having found, on repeated trials, that the light of a lamp, properly trimmed, 

 is incomparably more equal than that of a candle, whose wick continually grow- 

 ing longer renders its light extremely fluctuating, he substituted lamps for candles 

 in these experiments, and made such other variations in the manner of con- 

 ducting them, as might lead to a discovery of the resistance of the air to light, 

 were it possible to render that resistance sensible within the confined limits of 

 the machinery. 



Having provided 2 lamps, the one an Argand's lamp, which he 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 visible smoke, emitted only about Vt P'Tt 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 



