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



Let us take a specific case of an object uniform in re- 

 flexion factor and colour (hue and saturation), the visible 

 surface of which occupies a position in an approximately 

 vertical plane perpendicular to the line of sight. Let us 

 assume further that this object is viewed against a sky back- 

 ground uniform in brightness and colour. This condition is 

 shown in perspective in fig. 5. The eye or viewpoint is at A, 



Fig. 5. 



A Diagrammatical Illustration of the Relation of Object to Background. 



the line of sight being the line LS. The object, 2, is repre- 

 sented by the circle lying in the object plane abed, which 

 is approximately perpendicular to the line of sight. The 

 background, 1, is represented by the imaginary plane efgh, 

 also perpendicular to LS and situated at infinity. The angle 

 subtended by the object at the eye is 7, and is assumed to 

 be small. 



In fig. 6 is shown a cross-section obtained by passing a 

 horizontal plane through the line of sight. The object is 

 at Oj the line AB is the trace of the object plane and the 

 line M.N is that of the background plane. For the purposes 

 of this discussion we need deal only with that part of the 

 background immediately surrounding the object which is 

 included in the angle &>, that is the space lying within 

 the circle C and outside of circle E, of fig. 5, the pro- 

 jection of the object on the background plane. The 

 visual field in this case is as shown in the lower right-hand 

 drawing of fig. 6, the area marked 2 being the object and 

 that marked 1 the background. It should be understood 

 that the background is not limited as indicated, but that for 



