170 Capt. W. de W. Abney [Mar. 10, 



«, I" 2Z i f T A, '-A," . \ , 



^17 = at+a7+2^V — ■ ^h 4 "^ 



whence 



(A/-A/QI f (A/-A/Q3 , A> ,,J4^ 

 ~ A 1 ' + A 1 /, + 2&Z 1 IA/+A 1 " + 2^ 1 ^ 1+Ai; /2Z 1 



(A/-A/QI 4A/A 1 // + 2^ 1 (A 1 / +A/ / ) 4™ y 

 ~ A 1 , +A 1 " + 2/^ 1 + A 1 / +A 1 ,/ + 2M 1 * 2Z X ' 



For illustration, consider the simplest possible case : let b = & 1 = 

 |-A 1 , and Z> 2 = 6 3 = . . . = 0, and let 2&Zj be negligible ; we have — 



I -I = I cos + am*— . 

 and the equation of current will be — 



% = . j-Ism — - _-_| sln ^ • j,j j, 

 instead of the simple and familiar linear equation. 



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

 sphere." By Captain W. de W. Abney, R.E., F.R.S. 

 Received February 17, 1887. 



(Abstract.) 



The observations were made by means of the colour photometer 

 which General Festing and himself introduced last year, and which 

 they described in the Bakerian Lecture for 188G. They extended 

 over more than a year, the object being to ascertain the intensity of 

 the different rays in the solar spectrum after passing through various 

 thicknesses of the atmosphere. Owing to the unpromising results 

 obtained by Langley with his bolometer experiments, it was not 

 anticipated that the variation in the intensities of the different 

 rays would obey any law, but subsequent investigation showed that 

 as a rule the intensity of any ray obeyed the law enunciated by Lord 

 Rayleigh, in that I' = I e^***"*, where I and I' are the initial and 

 transmitted wave-lengths, x the thickness of the medium through 

 which the ray passed, and k a constant, X being the wave-length. The 



