for the Measurement of Visibility of Objects. 115 



the purposes of this discussion only that portion of it which 

 surrounds the object need be considered. It will be con- 

 venient for the sake of reference to divide the space into 

 two parts : (a) the space lying between A and the object 



Fiff. 6. 



Horizontal Section through the Line of Sight. 



plane will be termed the " foreground space," D ; (b) the 

 space between the object plane and the background plane 

 will be called the " background space/' K. 



Material particles in these spaces may react in four ways 

 upon light traversing this space. If very small, such particles 

 will cause diffraction ; if larger, reflexion, absorption, and 

 refraction. Thus the intensity of a beam of light travelling 

 in a given direction may be diminished by any one of these 

 four effects, also light may appear to originate in the space 

 between the point of observation and the object viewed from 

 any one of three of these four causes. It is unnecessary for 

 the purposes of this discussion to evaluate the losses or gains 

 in brightness for each of these different causes, it being suffi- 

 cient to evaluate the total gain or loss under any particular 

 condition. From the standpoint of apparent brightness any 

 diminution in the brightness value may be regarded as due 

 to absorption whether the light is absorbed or scattered by 

 the particles present in the space lying between the point of 

 observation and the surface being considered. Likewise any 

 increase in the apparent brightness may be regarded as a 



I 2 



