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



flux which enters the eye from the projected space imme- 

 diately surrounding that object. The sensation caused by 

 the incidence of radiant energy, which we call light, upon 

 the retina of the eye may be said to consist of two factors, 

 brightness and colour, the former being dependent upon the 

 intensity and the latter upon the quality of the incident 

 radiation. This second factor of the sensation may be said 

 also to consist of two parts, hue and purity or saturation. 

 Hue refers to the position, in the spectrum, of the dominant 

 wave-length, and saturation expresses the proximity of the 

 colour to monochromatism. It is evident, therefore, that a 

 sensation due to the impingement of radiant energy upon 

 the retina may vary in three respects, that is, with respect 

 to brightness, hue, and saturation. A contrast in the visual 

 field resulting in the visibility of an object may be due, 

 therefore, to brightness contrast, to hue contrast, or to 

 saturation contrast ; or to a combination of any two or all 

 three of these factors. 



For the purposes of the theoretical treatment of this 

 problem it will be necessary to make certain simplifying 

 hypotheses. Begin first with the problem of the deter- 

 mination of the visibility of an object uniform in colour 

 and brightness viewed against a background also of uniform 



Fiff. 1. 



-^sis^^E^i _ i~ 



Q 1 ■ 'f 



Diagrammatic Illustration of the Relation of Object to Background. 



colour and brightness. In fig. 1 this case is shown in per- 

 spective, 1 being the background, 2 the object, and A the 

 eye or view-point. The visual field will appear as shown in 



