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



the reflexion factor of the object for a fixed value of the 

 weather coefficient. It will be again noted that V 6 = when 



This completes the theoretical treatment of visibility due 

 to brightness contrast. In closing the discussion it may be 

 well to review briefly the most important points. Visibility 

 due to brightness contrast, Y b , is measured by a determina- 

 tion of B^, the veiling glare which when superposed upon 

 the object and background will reduce the contrast to a just 

 perceptible value. The fundamental equation is 



BH-B u==c 



where c is a constant depending for its value upon the sensi- 

 bility functions of the eye. Formulations of the functions 

 V 6 =/(-Ks) and Y b =f(W) lead to equations. (11) and (12), 

 by solution of which curves given in figs. 3 and 4 are 

 obtained. It is also shown that for Y b = i R 2 = W. As 

 stated previously, the term Y q is composed of two factors, 

 V*, hue visibility, and Y 8 , saturation visibility. Notwith- 

 standing the fact that the sensibility of the eye to hue and 

 purity differences is fairly well known, it is not possible to 

 formulate directly the visibility functions in these cases. 

 It may be said in general that V^=/(H 1 , H 2 ), but just what 

 form the function will take it is impossible to say. It is 

 probable that the maximum visibility will be found when Hj 

 and H 2 are complementary hues, diminishing to a zero value 

 as Hi — H 2 approaches zero. It is equally impossible in the 

 light of our present knowledge to evaluate the expression 

 Y s =f(S^ So). However, it is evident that Y s can be zero 

 only when Si = S 2 , that is, when the saturation of the object 

 is equal to the saturation of the background. It will pro- 

 bably be found that V s is directly proportional to the 

 saturation difference, in which case the expression would be 

 of the form, V s = a(S 1 — -S 2 ), where a is a constant of pro- 

 portionality which may or may not vary for different values 

 of H. At present it is sufficient to give the general form of 

 the evaluation of Y q , which includes Y h and Y s . The total 

 visibility, V, can be measured by the superposition of a 

 veiling glare, B y ; and by measuring B 1 and B 2 , Y b can be 

 computed by means of the theoretical equations. In this way 

 Y b and Y q may be separated if desired. It is evident that 

 the expression for V (total visibility) must be of the form 



V=/(V„ Y h , Y,). 



