SENSATION AND SENSE- ORGANS. 503 



directly that it arouses more sensation, and yet we know cer- 

 tainly that it does not. With a body of a given brightness 

 the amount of change produced in the end organs of the eye 

 will depend on the size of the image formed in the eye, pro- 

 vided the same part of its sensory surface is acted upon. 

 Now the size of this image depends on the distance of the 

 object; it is smaller the farther off it is and greater the 

 nearer, and measurements show that the area of the sensitive 

 surface affected by the image of the rising moon is no larger 

 than that affected by it when overhead. Why then do we, 

 even after we know this, see it bigger ? The reason is that 

 when the moon is near the horizon we imagine, unconsciously 

 and irresistibly, that it is farther off; even astronomers who 

 know perfectly well that it is not, cannot help forming this 

 unconscious and erroneous judgment and to them the moon 

 appears in consequence larger when near the horizon, just as 

 it does to less well-informed mortals. In fact we have a con- 

 ception of the sky over which the moon seems to travel, not as 

 a half sphere but as somewhat flattened, and hence when the 

 moon is at the horizon we unconsciously judge that it is 

 farther off than when overhead. But any body which ex- 

 cites 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 de- 

 clare that we see it so. 



So, again, a small bit of pale 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 walking 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. 



