444 Transmission of Sunlight through the Atmosphere. [May 5, 



I. " Transmission of Sunlight through the Earth's Atmo- 

 sphere. Part II. Scattering at Different Altitudes." By 

 Captain W. de W. Abney, C.B., D.C.L., F.R.S. Received 

 April 7, 1892. 



(Abstract.) 



In this paper the results of observations made by exposing platino- 

 type paper are recorded, and it is shown that the total intensity 

 of light as thus registered is the same as if observations had been 

 made on a ray of X4240 alone. The observations were made 

 at altitudes varying from sea-level to 12,000 feei, in different 

 countries, at different times of the year, and during four to five years. 

 The instrument in which the exposures were made is described, as 

 also the method of deriving the intensity of light from the developed 

 prints. The results of these observations agree closely with those 

 obtained by the measures of the spectrum which was described in 

 Part I of this subject. The value of h in the formula (1) I' = e""**~** 

 (from which can be calculated the loss of intensity of a ray of any 

 particular wave-length) was found to be 0"00146 at sea -level. It 

 was also found that k apparently varied as h 2 , h being the baro- 

 metric pressure. A table is attached, showing the value of the trans- 

 mitted light in the formula (2) I' = la*, where a is a constant and 

 x the air thickness in terms of the vertical thickness, jx being the 

 formula I' = Ie - ^, from which (1) and (2) are both shown to be 

 derived. 



Bar. 





a. 



Bar. 



Iff 



a. 



in inches. 



A 4 - 



in inches. 



30 



0-154 



0-856 



24 



0-098 



0-908 



29 



0-144 



0-866 



23 



0-090 



0-915 



28 



0-134 



0-875 



22 



0-083 



0-922 



27 



0-124 



0-884 



21 



0-075 



0-928 



26 



0-115 



0-891 



20 



0-068 



0-934 



25 



0-107 



0-899 



19 



0-062 



0-940 



