no. 3 



ATMOSPHERIC SCATTERING OF LIGHT FOWLE 



The coefficients a„\and a^xwere then subjected to a second refining 

 process. Following" the lead of Rayleigh, Schuster and L. V. King, 

 a a \ may be placed equal to er h where c is the base of the natural 

 logarithms. Then 



32 f 

 3 



{l(„-i). $-. + «/£ + D 



where ;/ is the index of refraction of air ; H, the height of a " homo- 

 geneous atmosphere " in cm. when the pressure p is 76 cm. H equals 

 7.99 X io 5 ; p, the observed atmospheric pressure ; A. the wave-length 

 in cm. ; N , the number of molecules per cm. 3 at 76 cm. pressure and 

 0° C; b, a factor to represent the amount of energy absorbed and 

 changed into heat and which approximates zero in the region con- 

 sidered (no selective absorption) ; D, a coefficient of transmission 

 suitable to whatever dust may be present. This dust is presumed to 

 be composed of particles so large that D is invariable with the wave- 

 length. 



Treating N and D as the unknowns, least-square solutions were 

 made by Miss F. A. Graves from the values of a„\ grouped year by 

 year. Table 3 contains the results. 



Table 3 



A r , the number of molecules per cm. 3 , 76 cm. pressure, 0° (.'. 

 D, the value of K for dry atmospheric dust. 



191O-II. . 

 I9I3..... 



1914 



1915 



1916 



No = (2.7s ± 0.02) io'° 



(2.69 ± 0.03) IO 10 



(2.66 ± 0.05) IO 19 



(2.74 ±0.05) IO 10 



(2.89 ±0.08) IO 19 



Weighted mean (2.72 ±0.01) io 10 



D = 0.005 ± 0.002 

 0.026 ± 0.003 

 0.010 ± 0.006 

 0.0 10 ± 0.005 

 0.032 ± 0.007 



Weight 18 

 6 

 3 

 3 

 1 



(The separate values were weighted inversely as the squares of their probable errors.) 



First to be noted is the close agreement of the mean value of the 

 number of molecules per cm. 3 with what is probably the best value ' 

 obtained from other methods (2.705 ±0.003) io 19 . The correspond- 

 ing value of Avogadro's constant is 6.09 X io 23 . Next to be noted are 

 the dust-transmission values. Remembering that a,i = e D , during 1910 

 to 191 1 a,i equals 0.995. That is, only about 0.5 per cent of the in- 



1 Millikan, Philosophical Magazine, 34, p. 3, 1917. See also " The Physical 

 Properties of Colloidal Solutions," Burton, p. 38, 1916, for table of determina- 

 tions of these constants by various methods. 



