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CAPTAIN W. DE W. ABNEY ON THE TRANSMISSION 
and air-thickness. This latter problem is one which astronomers have long studied, 
and their researches point out that, if 1 and I be the values of the light before and 
after traversing an air-thickness, 6 be the zenith distance, and a the constant of 
transmission, then V = Ia -sec e . 
Diagram 1. 
Owing to refraction, the formula a~ £ece cannot apply accurately where 6 is very large. 
This was pointed out by Bouguer, who calculated the variation of the thickness 
of the atmosphere traversed at different zenith distances, allowing for refraction. 
This calculation was subsequently verified by Professor Forbes, and the results 
are tabulated in his Bakerian Lecture “ On the Transparency of the Atmosphere 
and the Law of Extinction of the Solar Bays in passing through it,” which appeared 
in the ‘ Phil. Trans.,’ 1842, Part II. Since this volume may be difficult to refer to, I 
quote part of his Table to show these variations. 
