ga Count Rumford's Account of a Method of 
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 my letter by inserting a particular ac- 
count of them. In general they all conspired to shew 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 I 
was by no means surprised that its want of transparency could 
not be rendered sensible in the small distance to which my ex- 
periments were necessarily confined : but still I think means 
may be found for rendering its resistance to light apparent, 
and even of subjecting that resistance to some tolerably accu- 
rate 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 clear weather, would probably 
lead to a discovery of the real amount of the resistance of the 
air to light. 
Of the Loss of Light in its Passage through Plates or Panes of 
different Kinds of Glass. 
in these experiments I proceeded in the following manner. 
Having provided two equal Argand s lamps, A and B, well 
trimmed, and burning with very clear bright flames, they 
