210 ILLUSIONS EXPLAINED. 



ferent distances and in different directions, so that the im- 

 age of a dome is formed on the retina of our eyes when 

 there is nothing in external nature to conform to it." 



Thus the vaulted appearance of the sky is the creation 

 of our senses, qr, rather, of our minds under the illusive 

 evidence of our senses. The vault forms itself over our 

 heads wherever we are, and we carry it with us wherever 

 we go. Each person has his own sky, corresponding. to 

 his own position, wherever it is, and it is a different one 

 from that of any person who is in any different position. 

 There may be many objects common to both, and those 

 which are at a great distance may be very neai'ly in the 

 same relative position, but they are really different ; so 

 that, wherever we go, our senses form for us continually 

 an ever-changing sky over our heads, in which the objects 

 appearing in it that are comparatively near, such as the 

 clouds, kites flying, birds, and the outlines of distant 

 mountains, or the summits of spires tall enough to appear 

 in the sky, as we move, continually change their relative 

 positions, and some of them finally disappear, while other 

 objects come into view to take their places. In respect to 

 these various objects, there is for each a reality which pro- 

 duces the image of it in our minds ; but as to the vaulted 

 appearance of the form which the assemblage of them as- 

 sumes, it is all an illusion. 



The nature of the illusion is partly explained by the fact 

 that objects appear smaller at a distance than when near. 

 Thus, of two ships, the mast of one which is near appears 

 much taller than the one which is at a distance that is, 

 the top mavks a much higher point in the sky, though the 

 one may be really no taller than the other. 



We see this plainly illustrated by the masts of the two 

 whale ships in the engraving on the opposite page. 



It results from this principle that, in the case of an ob- 



