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



Since ^B = B! — B 2 or B 2 — B x , depending upon the absolute 

 magnitude of the values, it follows that 



that ratio being used which results in a value of k greater 

 than unity. 



It is evident that if Bj __ , 



the object, 2, of fig. 1 will be just visible against the back- 

 ground, 1 ; 



while if ^ < k, 



-Do 



the object will be invisible. 



Now, in case B x , 



if it is possible by any means to reduce the value of the ratio 

 to the point where ^ 



zl! 



B 2 



is just less than k, the object will be rendered just invisible. 

 This can be accomplished by adding the same brightness, B r , 

 to both B x and B 2 . B v being of the magnitude required to 

 satisfy the equation 



Bj + B, 



B 2 + B„ 



= k. 



This superposed brightness, B v , will be referred to as 

 u veiling glare/'' 



Now, the magnitude of B^, required to satisfy the equation 

 just given may be taken as a direct measure of the visibility 

 of objects under constant conditions of illumination. It is 

 evident, however, that the value of B v required by the equa- 

 tion will depend not only upon the ratio of B x to B 2 , but also 

 upon the absolute values of those terms. Now, assuming 

 that k is constant, and not dependent upon B 1? visibility 

 must be independent of the absolute value of either B x or 

 B 2 , and a function only of their ratio. Such evaluation may 

 be accomplished by writing, 



V6 Bi- 

 In order to determine visibility, therefore, it is only 



