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IX. Transmission of Sunlight through the Earth's Atmosphere. 
By Capt. W. de W. Abney, R.E., F.R.S. 
Received February 17,—Read March 10, 1887. 
It will be in the recollection of the Royal Society that early last year General Festing 
and myself brought forward a method of colour photometry, which is described in a 
paper in the ‘Phil. Trans.,’ 1886. 
In a postscript, dated June, we added a description of a somewhat modified 
apparatus in which any variation in the intensity of the light producing the spectrum 
under measurement was equally reproduced in the comparison-light. 
This was effected by causing a part of the light which passed through the slit of 
the spectroscope to act as the comparison-light. For the sake of easy reference the 
diagram of the modified apparatus is reproduced in this paper. 
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