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



as quality contrast,, G q . It should be borne in mind, how- 

 ever, that this term includes two independent variables, both' 

 of which must be considered in any evaluation of visibility 

 due to quality contrast. It is entirely possible that the visi- 

 bility due to Ca and C s could be evaluated separately, but 

 this would require a large amount of fundamental research 

 which it will not be advisable or necessary to go into at 

 this time. 



The visibility resulting from brightness contrast is directly 

 proportional to the subjective contrast and hence for an eye- 

 adapted to a fixed brightness level to the ratio of the two- 

 brightnesses, B 1 and B 2 . Therefore, we may write 



-AS) 



A direct measurement of B 1 and B 2 will therefore determine 

 Vj. This method is satisfactory and applicable if there is no- 

 hue or saturation contrast between object and background, 

 and if the object and background are each uniform in bright- 

 ness, hue, and saturation. These conditions, however, do not 

 exist in practice. Brightness measurements in the presence 

 of colour differences are difficult to make and subject to great 

 errors. Moreover, in practice it is frequently necessary to- 

 determine visibility in cases where the object or background 

 is not uniform in brightness, the variation being irregular 

 and practically indeterminate. In such cases the determina- 

 tion of the effective values of B x and B 2 is extremely difficult 

 and, if not impossible, entirely impracticable. These objec- 

 tions to this method of measuring visibility are so serious as 

 to make it almost useless from the practical standpoint, and 

 hence it is necessary to consider other methods for the 

 accomplishment of the desired result. 



A careful consideration of the causes of lowered visibility 

 reveals the fact that loss of visibility results from the intro- 

 duction of veiling glare between the eye and the object and 

 also to a lowering in the brightness of the object and its 

 background. In order to explain the action of this veiling 

 glare in producing a diminution or in some cases an entire 

 loss of visibility, it will be helpful to present a curve, fig. 2,. 

 which shows in graphic form the contrast sensibility of the 

 eye *. The abscissae values of this curve are the field or 

 background brightness, the ordinates are the ratio of AB to 

 B x . The term /\B, the least perceptible difference, is the 

 brightness difference between B x and B 2 when that difference 



* P. G. Nutting, Trans. 111. Eng. Soc. vol. xi. p. 945 (1916). 



