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



position of the members relative to the axis of sight. It is 

 not possible to obtain exact constancy of B^ for all values of 

 Bj, but B* can be kept within the range for which k is con- 

 stant. However, in case B^ should vary beyond the specified 

 range of values it is still possible to compute B P , provided 

 the resulting change in the value of k is known. If B^ is 

 measured, which can easily be done, the corresponding values 

 of k may be read from the curve in fig. 2. In practice it is 

 found that B^ can be kept within the required limits in almost 

 all cases by choosing a wedge of proper density gradient and 

 by adjusting the number of teeth on the pinions so .that the 

 desired relative motions of S and W are obtained. 



The above consideration shows that it is not necessary to 

 -exactly simulate in the instrument the relations existing in 

 nature between the values of the veiling glare brightness 

 and the opacity arising from the particles suspended in the 

 foreground space. Since this relation is not constant for all 

 natural conditions, being dependent up the nature, size, and 

 spatial distribution of such scattering and absorbing particles, 

 it would be quite impossible to make a single instrument 

 exactly simulating all possible conditions resulting in lowered 

 visibility. 



Now it will be noted by referring to the theoretical treat- 

 ment that 



V= — 



V 



It is necessary, therefore, to determine the value of B x . This 

 is done by substituting for the glare field X, in fig. 7, the 

 photometric field Y. The instrument being calibrated as a 

 brightness photometer, the value of B x is read directly from 

 "the scale when a photometric balance exists between the fields 

 ■c and d. The field d is filled by the image of the background. 

 In order to obtain a value of W, the weather coefficient, 

 which is defined by the expression 



w =t' (») 



it is necessary also to determine the value of E 2 , the illu- 

 mination on the object plane. This may be done by 

 measurement of the brightness, B , of a surface on the 

 object plane of which the reflexion factor R is known. 

 Such a surface is termed a test plane and is made by 

 covering a frame of the proper size with canvas or sail cloth 

 painted with several coats of a matt white paint. The re- 

 flexion factor, R , of this surface is carefully determined by 

 suitable laboratory methods. When determinations of visi- 

 bility are to be made this test plane is fixed in the object 



