186 THE HUMAN BODY 



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 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 

 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. 



Erroneous perceptions of this sort are known as sensory illu- 

 sions; and we ought to be constantly on guard against them. 



