THE RECEPTOR SYSTEM 221 



loudiiess in each ear is the same. Hence we are led, by mental 

 laws outside of the physiological domain, to suspect that its cause 

 is not in our Body, but outside of it; and depends not on a condi- 

 tion of the Body but on something else. 



Sensory Illusions. "I must believe my own eyes" and "we 

 can't always believe our senses" are two expressions frequently 

 heard, and each expressing a truth. No doubt a sensation in 

 itself is an absolute incontrovertible fact: if I feel redness or hot- 

 ness I do feel it, and that is an end of the matter: but if I go be- 

 yond the fact of my having a certain sensation and conclude from 

 it as to properties of something else if I form a judgment from 

 my sensation I may be totally wrong; and in so far be unable to 

 believe my eyes or skin. Such judgments are almost inextricably 

 woven up with many of our sensations, and so closely that we 

 cannot readily separate the two; not even when we know that 

 the judgment is erroneous. 



For example, the moon when rising or setting appears bigger 

 than when high in the heavens we seem to feel directly that it 

 arouses more sensation, and yet we know certainly 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, provided 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 t hat- 

 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 mor- 

 tals. In fact we have a conception 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 excites the same extent of the sensitive surface of the eye 

 at a great distance that another does at less, must be larger than 



