SEtfSOBT ILLUSIONS. 47? 



modifiable by any particular movement, and so appear to 

 us rather as mental states, pure and simple, than bodily 

 sensations. 



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

 "we can't always believe our senses" are two expres- 

 sions frequently heard, and. each expressing a truth. No 

 doubt a sensation in itself is an absolute incontrovertible 

 fact: if I feel redness or hotness I do feel it and that is an 

 end of the matter: but if I go beyond 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 bigger 

 the nearer, and measurements show that the area of the 

 sensitive surface affected by the image of the rising moon 

 is no bigger 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 

 astronomerswho know perfectly 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 conception of the sky over which the moon trav- 

 els, 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 



