VOL. LXXXIV.] PHILOSOPHICAL TRANSACTIONS. SSj 



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. Hence, 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 diminution of the light arising from the imperfect transparency of 

 the air should not be perceptible within the limits of that distance. Bat if, on 

 the contrary, it should be found on repeated trials, that the equilibrium was re- 

 stored when the greater light had arrived at a distance short of 202 inches, it 

 might thence be concluded, that such effect might safely be attributed to the 

 imperfect transparency of the air : for though the light of the smaller lamp would 

 of course be diminished as well as that of the greater; yet as there is every rea- 

 son 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 augmen- 

 tation of the distance 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 ad- 

 ditional distance, amounting to more than 100 inches, it is evident that the di- 

 minution of the light of the greater lamp, arising from the imperfect transparency 

 of the medium, must be greater than the diminution of the less lamp, arising 

 from the same cause ; and consequently that the effects of such diminution would 

 become apparent in the experiment, were they in reality considerable. 



Having made a number of experiments with this view ; the results of them, so 

 far from affording means for ascertaining the resistance of the air to light, did 

 not even indicate any resistance at all ; on the contrary, it might almost be inferred 

 from some of them, that the intensity of the light emitted by a luminous body in 

 air is diminished in a ratio less than that of the squares of the distances ; but as 

 such a conclusion would involve an evident absurdity, namely that, light 

 moving in air, its absolute quantity, instead of being diminished, actually goes on 

 to increase, that conclusion can by no means be admitted. 



Besides the experiments above mentioned, a great number of others similar 

 to them were made, and with the same view, with nearly the same results ; and 

 in general they all conspired to show that the resistance of the air to light was 

 too inconsiderable to be perceptible ; and that the assumed law of the diminution 

 of the intensity of the light may with safety be depended on. 



That the transparency of air in its purest state is very great, is evident from 

 the very considerable distances at which objects, and such even as are but faintly 

 illuminated, are visible ; and it was not surprizing that its want of transparency 

 could not be rendered sensible in the small distance to which the experiments 

 were necessarily confined : but still he thinks that means may be found for ren- 

 dering its resistance to light apparent, and even of subjecting that resistance to 



