for the Measurement of Visibility of Objects. 129 



glare, the curve having log B w values as ordinates and scale 

 readings as abscissae. Curve B is for brightness. It has 

 scale readings as ordinates and brightness values as abscissa?. 

 By the use of Si and Curve A, B w is obtained, and by using 

 Carve S 2 and Curve B, the value of B t is found. S 3 in 

 conjunction with Curve B gives B . 



The relations from which the Curves A and B are obtained 

 and the methods of computing the final values of visibility 

 from the observed values are given below. The values read 

 directly are the scale readings : 



S : = Visibility setting, 

 So = Photometric setting on background, 

 S 3 = Photometric setting on test plane. 

 By the use of the Curves A and B : 



Si gives B u = Brightness of veiling glare, 



5 2 gives B x = Brightness of background, 



5 3 gives B = Brightness of test plane. 



Other terms, the values of which must be previously 

 determined, are : 



T a = Transmission of wedge, W, 



T m = Transmission of mirror, M, 



R Hi = Reflexion factor of mirror, M, 



R = Reflexion factor of test plane, 



B c =Brightnessof uppersurfaceof diffusing member, C. 



The terms of which the values may be computed from the 

 available data are : 



V = Total visibility, 



W = Weather Coefficient, 



B< = Total field brightness, 



B/ = Brightness of glare field instrument. 



E 2 = Illumination on object plane. 



Some of the relations existing between the various terms 

 are given below : 



p B r ' 



v ~t~t~' 



B v ' = B c . R,„, 



B — c ' m /io\ 



■*-a ' -*-m 



Phil Mag. S. 6. Vol. 39. No. 229. Jan. 1920. K 



