4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I23 



represented by the sum of the wavelength-independent term and the 

 term in the reciprocal of the square of the wavelength should be 

 relatively large, while on clear days it should be correspondingly 

 small. However, the distribution between the two terms appears to 

 depend rather sensitively on the measured values of the transmission 

 coefficients, and the computed value of one or the other term may be 

 negative. Such negative values are probably without physical signifi- 

 cance since they may arise from the normal experimental error in the 

 observations. 



It seems important to emphasize that so far as the determination 

 of ozone itself is concerned, the object of the method is to approximate 

 as closely as possible the background optical densities as they would 

 be in the absence of ozone absorption, without particular regard as 

 to how these came about, that is, from molecular scattering, haze, etc. 

 The additional information contained in 8 and C (see equation (i) 

 below) is from this point of view a byproduct, but from the point of 

 view of the state of the sky it may be as interesting as the ozone 

 determination. 



In the interests of simplicity and rapidity of calculation it would 

 be helpful to limit the number of unknowns to as small a number as 

 sufficed for the ozone determination. From what has been said above, 

 the optical density of the atmosphere at any particular wavelength 

 can, for our purposes, probably be expressed, in the absence of other 

 selective absorption, as the sum of a term representing the ozone 

 absorption, a term representing the pure Rayleigh scattering by the 

 molecules of the atmospheric gases themselves, a term, just discussed, 

 representing the scattering by particles of haze of intermediate size, 

 and finally a wavelength-independent term representing the scattering 

 by large particles of haze. 



We should like to emphasize here that, with the method of calcula- 

 tion to be described below, an additional term in another power of the 

 wavelength with unknown coefficient could be introduced without 

 excessive increase of the computational work. Observations at addi- 

 tional spectrum places might be required to justify the use of such a 

 term, but this, too, would not mean an excessive increase in the compu- 

 tations. If work such as the present were attempted at lower altitudes, 

 a term of this kind should permit a closer approximation to the 

 scattering, which would in general be more intense there. 



The value for the Rayleigh-scattering term for dry air can be 

 calculated^ (see section 6). The other terms in the total optical 



^ See footnotes 2 and 6. In equation (3) of Tien Kiu (footnote 6) we believe 

 that the coefficient of p in the denominator was intended to be 7. 



