WHAT ANIMALS SEE 



WRITING in Land and Water on the sleep of fish, 

 Mr Matthias Dunn accounts for the transition state 

 between sleep and waking observed in fish by the 

 existence of a dual nervous system, and a double 

 use of the eye 'the under eye being closed, while 

 they watch their enemies with the outside eye.' The 

 first supposition would account for the power which 

 most animals have of passing instantaneously from 

 sleep to wakefulness or of ' sleeping with one eye 

 open/ as the phrase is. The second is somewhat 

 misleading, supposing, as it does, that the eye of 

 the fish, which is flatter and less perfectly fitted as 

 an organ of sight, can perform functions which, even 

 in the eye of the higher vertebrate, are mechanically im- 

 possible. What animals see, and what they do not, 

 is by no means easily discovered. The compound 

 eyes of insects present an image pieced together like 

 mosaic, each portion being conveyed by a separate 



facet of the eye. But a much more interesting 



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