402 THE BODY AT WORK 



our eyes. Hence, as Francis Bacon pointed out, a picture 

 appears more real when one eye is closed than when both are 

 open. Its middle distance at once falls back. 

 Innumerable are the illustrations which may be given of 

 errors of sensory judgment, but none are more striking than 

 the various figures which may be drawn with converging or 

 diverging lines. The mind under-estimates acute and over- 

 estimates obtuse angles. It is impossible to convince oneself 

 that in Fig. 36 the line A bisects a symmetrical arch. Equally 

 difficult is it to believe that in Fig. 37 the line with diverging 

 terminal segments and the line with converging terminal seg- 

 ments are of exactly equal length. In the Ruskin Museum 



FIG. 37. Two HORIZONTAL LINES OF EQUAL LENGTH THE ONE WITH DIVERGING, THE 

 OTHER WITH CONVERGING, TERMINAL LINES. 



at Sheffield there is a sketch by the master of the fagade of a 

 church which shows a vertical tower to one side of a triangular 

 pediment, or, rather, this is what the sketch was meant to 

 show, and does show, when measured on an architect's table. 

 In effect the tower appears to be leaning towards the pediment. 

 Errors of judgment of this type have been attributed to the 

 curvature of the lines of a rectilinear image on the retina, the 

 mind judging the distance between two points by the length 

 of the chord, and not the length of the arc which joins them. 

 This is very simply illustrated by the example of the apparently 

 greater length of a filled space than of a vacant one. 



ABC 



A B looks longer than B C. If A B C be represented as a 

 curved line, the arc A B will, of course, be longer than the 

 chord B C. But it is not safe to suppose that the mind com- 

 pares the length of an arc with the length of a chord. Judg- 

 ment is based upon experience, and probably the illusion is 

 due to more subtle causes than the curvature of the retina. 



