22 Experiments on the Relative Intensities 



With regard to these experiments, it may be ob- 

 served that were the resistance of the air to Hght, or 

 the diminution of the hght from the imperfect trans- 

 parency of air, sensible within the hmits of the incon- 

 siderable distances at which the candles were placed 

 from the photometer, in that case the distance of the 

 two equal lights united ought to be to the distance 

 of one of them single in a ratio less than that of the 

 square root of 2 to the square root of i. For if the 

 intensity of a light emitted by a luminous body, in a 

 space void of all resista^ice, be diminished in the pro- 

 portion of the squares of the distances, it must of 

 necessity be diminished in a still higher ratio when 

 the light passes through a resisting medium, or one 

 which is not perfectly transparent ; and from the dif- 

 ference of those ratios, — namely, that of the squares of 

 the distances, and that other higher ratio found by 

 the experiment, — the resistance of the medium might 

 be ascertained. This I have taken much pains to do 

 with respect to air, but have not as yet succeeded in 

 these endeavours, the transparency of air being so 

 great that the diminution which light suffers in pass- 

 ing through a few inches or even through several feet 

 of it is not sensible. 



Having found upon repeated trials that the light of 

 a lamp, properly trimmed, is incomparably more equal 

 than that of a candle, whose wick continually growing 

 longer renders its light extremely fluctuating, I sub- 

 stituted lamps to candles in these experiments, and 

 made such other variations in the manner of conduct- 

 ing them as I thought bid fair to lead to a discovery 

 of the resistance of the air to light, were it possible to 



