26 Experiments on the Relative Intensities 



almost be inferred from some of them that the inten- 

 sity 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, actu- 

 ally goes on to increase, that conclusion can by no 

 means be admitted. 



Besides the experiments above mentioned, I made a 

 great number of others, similar to them, and with the 

 same view ; but, as their results were all nearly the 

 same, I have not thought it worth while to lengthen 

 this paper by inserting a particular account of them. 

 In general, they all conspired to show that the resist- 

 ance of the air to light was too inconsiderable to be 

 perceptible, and that the assumed law of the diminu- 

 tion 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 dis- 

 tances at which objects, and such even as are but 

 faintly illuminated, are visible ; and I was by no means 

 surprised that its want of transparency could not be 

 rendered sensible in the small distance to which my 

 experiments were necessarily confined. But still I 

 think means may be found for rendering its resistance 

 to light apparent, and even of subjecting that resist- 

 ance to some tolerably accurate measure. 



An accurate determination of the relative intensity 

 of the sun's or moon's light, when seen at different 

 heights above the horizon, or when seen from the top 

 and from the bottom of a very high mountain, in very 



