288 NATURE'S TEACHINGS. 



seems to take place quite suddenly with some individuals, as if 

 the basin were flexible, and were suddenly turned inside out, it 

 occurs more gradually with others, the concavity slowly giving 

 way to flatness, and the flatness progressively rising into con- 

 vexity. 



" Not unfrequently, after the conversion has taken place, the 

 natural aspect of the object continues to intrude itself, some- 

 times suddenly, sometimes gradually, and for a longer or shorter 

 interval, when the converse will again succeed it as if the 

 new visual impression could not at once counteract the previous 

 results of recent experience. At last, however, the mind seems 

 to accept the conversion without further hesitation ; and after 

 this process has once been completely gone through, the observer, 

 on recurring to the same object, will not find it possible to see 

 it in any other than its converted form, unless the interval 

 should be long enough to have allowed him to forget its aspect. 



"Vagaries, however, sometimes occur in these experiments of 

 which it is difficult to give any certain explanation, but which 

 would be probably found referable to the same general principle, 

 if we were acquainted with all the conditions of its operation." 



THE MULTIPLYING-GLASS. 



STILL more extraordinary examples of the combining power 

 of vision are to be found in the eyes of spiders and insects, 

 more especially when we compare them with the work of man. 



BYBS OP 8PIDKB. MULTIPLYING-OLASS. 



If we take a common Multiply ing-glass, such as is shown in 

 the figure, and look at a flower or other object through it, we 

 see the object repeated as many times as there are different 

 foci of vision in the instrument. 



