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



T m and E,„.are constants, while B c and T tt are variable with 

 the value of the scale reading. The relation between the 

 scale reading and B c and R m must be determined, after 

 which equation (13) is used for obtaining the values of log B B , 

 from which in turn the Calibration Curve A is plotted. 



B r ' 



e -rI; • • (*) 



• • - B„ • 



B is a constant. Bx and B are determined from S 2 and S 3 

 in connexion with the Calibration Curve B. Therefore W 

 may be computed. W and V are thus determined and pro- 

 vide the necessary data for plotting the total visibility as a 

 function of the weather condition, which may then be com- 

 pared with the theoretical curves shown in fig. 3. If the 

 objects on which the measurements were made were of 

 the same colour as the background (the visibility being 

 entirely due to brightness contrast) those experimental 

 curves should coincide with the theoretical ones applying 

 to the same conditions. If a colour (either hue or satura- 

 tion) difference exist, the difference between the measured 

 and computed values will be the part of the total visibility 

 due to quality contrast. In this way the form of the 

 expression V ? -=/YW) ma}' be evaluated graphically, 



Bi-Bi'+fi 1 .(T..T, f ) (15) 



By computing the value of B t by this equation for various 

 scale readings its constancy or variation can be determined. 

 This will show, in connexion with fig. *2, whether or not k 

 remains constant and if not will provide the required data 

 for the determination of its value. Thus any error intro- 

 duced by a variation of k may be eliminated. 



Attention should again be called to the analogy between 

 the method employed in the instrument for reducing the 

 visibility to a zero value and the phenomenon of lowered 

 visibilit}' in nature. Given an object seen against the sky 

 and at some distance from the observer. Now let a cloud or 

 mist or any collection of small material particles be formed 



