for the Measurement of Visibility of Objects. 131 



in the foreground space of sucli concentration or density that 

 the object is just visible. In the terminology previously 

 adopted we m;iy state this condition by the equation 



(B.. TW+P, 



(B 2 .T,)+P,= C (lfi ) 



In the instrument the condition when a visibilit} r setting is 

 made is given by 



B 2 + B„ 

 B P ' 



C; 



where B,, = rp 



B,(T,.T,) + B. ' 

 B 2 (T .T ra ) + B„' 





(17) 



Equations (16) and (17) are of exactly the same form. 

 The term T^ in (16), which is the transmission of the fore- 

 ground space, is replaced in (17) by (T tt . T m ), the transmission 

 of the wedge and mirror of the instrument. The term P^ in 

 (16), which is the brightness dae to light reflected or re- 

 fracted by the particles in the foreground space, is replaced 

 in (17) by B/, the brightness of the glare field of the instru- 

 ment. The constant c is the same in both cases, being- 

 equal to 



k or j , 



depending upon whether B x is greater or less than B 2 . 



As was pointed out previously the exact relation between 

 the values of T and P varies greatly with the nature of the 

 particles producing the diffusion in the foreground space. 



Since the relation is subject to so much variation no 

 attempt was made in the design of the instrument to imitate 

 any particular kind of natural fog or haze. Another factor, 

 however, must be considered in fixing the relation of B ' to 

 (T a :T m ). this being the necessity for keeping the total tield 

 brightness (B i =LV + B 1 (T a .T OT )) practically constant for all 

 possible values of the scale reading. Another point requiring 

 consideration was the necessity for keeping the light source 

 sufficiently small, with respect to voltage and current con- 

 sumption, to permit of convenient operation by easily portable 

 batteries. 



A large amount of data relative to the visibility of objects 



K2 



