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Mr. W. Brennand. 



the barrel with aperture c sin o being directed to the Sun, the circular 

 darkened spots were found to be very accurately of uniform depth. 

 Further, I calculated the times of exposure, for a (particular) 

 mitrailleuse which ought, on the law i* cosec 0, to give a uniform 

 tint. I exposed this mitrailleuse for these calculated times, first in 

 the plane of symmetry, afterwards in a plane inclined to it at an 

 angle of 52 ; the results agreed closely with my anticipation, and 

 show to cosec 6 to be a very good approximation. 



22. I have therefore made full use of the expression t' cosec 

 for the chemical action of the light of the sky in a circle distant 

 from the Sun (whose altitude is ). 



In carrying out integrations which include the portion of the sky 

 actually occupied by the Sun, we do not, by employing this formula, 

 introduce any infinite expression ; for each circular band of the sky 

 of small breadth cd distant from the Sun has an area 2ir sin 9 dO\ 

 the chemical action of such band is therefore 2w u dO : so that the 

 total chemical action thus attributed to the sky in the area occupied 

 by the Sun's disk would be inappreciable. 



23. Bunsen and Roscoe (' Phil. Trans.,' 1859, p. 891) determined 

 chemically the action of the rays falling from a measui-ed portion of 

 cloudless sky situated near the zenith, and then compared the visual 

 luminosity of this same portion of zenith sky with that of the total 

 heavens. They say " the amount of light chemically measured, 



