?R64- 



for the Measurement of Visibility, of Objects. 125 



of visibility by a veiling glare alone, due to the fact that the 

 values of B 1 and B 2 are so high that a source of very high 

 intensity would be required to give the required value of B c . 

 As it is desirable to make the instrument as portable as pos- 

 sible such sources cannot conveniently be used on account of 

 the excessive weight of storage batteries required to operate 

 them. By using an absorbing wedge in the axis of sight a 

 much smaller lamp may be used. Such procedure does not 

 in any way interfere with the correct determination of B y . 

 In order to obtain the maximum possible illumination on the 

 diffusing member with a lamp of given energy consumption, 

 the interior walls of the chamber inclosing the source, S, are 

 painted white. This tends to increase the brightness of the 

 diffusing member and also to increase the uniformity of 

 illumination on this surface, which is extremely desirable. 

 This painting of the interior walls prevents the use of the 

 inverse square law in computing the illumination on 

 the diffusing surface from known values of intensity of 

 source and distance between source and surface. This, how- 

 ever, does not interfere in any way with the operation of the 

 instrument. 



For a given ratio of B x to B 2 the value of B v required to 

 produce a loss of visibility is directly proportional to the 

 absolute values of B x and B 2 . Thus, by reducing the appa- 

 rent brightness of B x and B 2 to one-tenth of their actual 

 values by means of the member W, only one-tenth of the 

 amount of veiling" glare from the surface of M will be 

 required to produce a given lowering of visibility. The 

 brightness of the glare field of the instrument will be 

 designated by By' and should not be confused with the 

 term B„ appearing in the equations. The value of B r is 

 computed from those of B v ' and T a , the transmission of the 

 wedge W at the point through which passes the axis of 

 sight. 



The statement that the value of B y , appearing in the funda- 

 mental equations, is directly proportional to B 3 or B 2 and 

 hence inversely proportional to T a rests upon a basic assump- 

 tion which should be mentioned at this point. In order for 

 this to be true, k, the contrast factor of the eye must remain 

 constant. That is, the total field of brightness to which the 

 eye is subjected must not change sufficiently to cause an ac- 

 companying change in k. This factor is satisfied in the 

 instrument by so adjusting the density gradient of the 

 wedge AY and the linear velocity of the wedge relative to 

 that of the source, that the total field of brightness B^, of 

 the instrument remains sensibly constant regardless of the 



