for the Measurement of Visibility of Objects, 119 



for the relation between these two terms can be given with- 

 out an exact knowledge of the physical characteristics of the 

 scattering and absorbing particles. Thus, in case the particles 

 are opaque and of low reflecting power, the term P may be 

 almost negligible while T is very low, so that the entire effect 

 upon the apparent brightness of an object viewed through 

 the space filled with such particles is due to the absorption 

 of light by the particles, the expression for apparent bright- 

 ness being L=B.T. On the other hand, if the particles 

 are highly reflecting or transparent, the predominant factor 

 may be P (the effective brightness). The relative magnitude 

 of P and T depends to some extent also upon the conditions 

 of illumination prevailing in the space considered. It is not 

 feasible, therefore, to formulate the relation between P and T 

 for all cases since this relation involves a consideration of 

 the nature, size, and spatial distribution of the scattering 

 particles and also the conditions of illumination. 



In closing this section of the discussion it will be well 

 to summarize briefly the conclusions reached regarding the 

 visibility of an object under natural atmospheric conditions 

 and let its visibility have an appreciable value which may 

 be termed its initial visibility. A change of this initial 

 value is in general due to the presence in the atmosphere of 

 material particles which may either diffract, absorb, reflect, 

 or refract the light travelling through the space occupied by 

 such particles. If these particles are confined to the back- 

 ground space the change in initial visibility may be either 

 positive, negative, or zero, depending upon the particular 

 conditions. If the particles exist in the foreground space 

 the change in the initial visibility will in o-eneral be either 

 zero or negative, although it is possible under certain con- 

 ditions to produce an increase in visibility by means of 

 absorbing elements in the foreground. This latter case is 

 very unusual. The final conclusion may therefore be drawn 

 that in the great majority of cases low visibility is due to the 

 presence of absorbing or scattering material in the foreground 

 space which operates in such a way as to lower the apparent 

 contrast between the object and its background. 



The Measurement and Specification of Visibility. 



The general principles upon which the specifications of 

 visibility must be based have been outlined in the section 

 dealing with the theory of the subject. It will be re- 

 membered that visibility may be specified in terms of the 

 brightness of a veiling glare, !>„, and the brightness of 



