478 THE HUMAN BODY. 



body which excites the same extent of the sensitive surface 

 of the eye at a great distance that another does at less, must 

 be larger than the latter; and so we conclude that the moon 

 at the horizon is larger than the moon in the zenith, and 

 are ready to declare that we see it so. 



So, again, a small bit of light gray paper on a white sheet 

 looks gray: but placed on a large bright green surface it 

 looks purple; and on a bright red surface looks blue-green. 

 As the same bit of gray paper is shifted from one to the 

 other we see it change its color: it arouses in us different 

 feelings, or feelings which we interpret differently, although 

 objectively the light reflected from it remains the same. 

 Similarly a medium-sized man alongside of a very tall one 

 appears short, but when Avalking with a very short one, tall. 



Such erroneous perceptions as these are known as sensory 

 illusions; and we ought to be constantly on guard against 

 them. 



