116 Mr. Loyd A. Jones on a Method and Instrument 



superposed brightness arising from any or all of the possible- 

 causes. The action of a given space containing material 

 particles upon a brightness viewed through that space may 

 effectively be defined by two factors, the transmission, T, 

 and the effective brightness, P. The transmission may be 

 defined as the ratio of the transmitted to the incident in- 

 tensity, and effective brightness as the apparent brightness 

 of a perfectly black surface when -viewed through the space 

 under consideration. Consider the case of a surface of which 

 the real brightness is B viewed through specified space of 

 w T hich the transmission is T, and the effective brightness P. 

 The apparent brightness, L, of that surface will be given by 

 the expression L= (B . T) 4- P. 



Turning now to a consideration of conditions existing in 

 nature, if neither the background nor foreground space con- 

 tained any material particles capable of producing a scattering 

 of light, the sky would be perfectly black and B 1} the bright- 

 ness of the background, would be zero, while the apparent 

 brightness of the object would be equal to its real bright- 

 ness, L 2 = B 2 , Such conditions, of course, never exist since 

 the air itself and the very small particles of matter always 

 carried in suspension cause scatter by diffraction, thus giving 

 rise to the blue light that we call the sky. Such conditions 

 we term a perfectly clear atmosphere, the sky being a dark 

 blue, practically uniform in colour and brightness from zenith 

 to horizon. There is practically no emission or absorption 

 of light within the foreground space, hence L^Bj and* 

 L 2 = B 2 = E 2 .R^- 

 Consider next the case where the background space, K, 

 contains, in addition to the scattering material always 

 present, larger particles which produce scatter by reflexion 

 or refraction or both ; Lx^B, . T*) + P*. In most cases the 

 fog or clouds formed by the particles in the space K are so 

 dense that T^ is very small and L x is practically = P A . That 

 is, the sky is obscured and the entire brightness is due to light 

 reflected and refracted from the cloud structure within the 

 background space. Since there is no scattering material 

 within the space D, L 2 = B 2 . Finally, assume that both 

 space D and K are filled with scattering particles, then 



It is unnecessary in pi'actice to separate the factors of the 

 brightness due to the various causes in the background space, 

 and hence for the sake of simplicity the symbol Bj (back- 

 ground brightness) is used to include all such factors. The 

 symbol B L will therefore be used for the term (B 1 . T a + P/t). 



