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by a scarred tissue which is unfit for the process of seeing. Now 

 our physical knowledge would lead us to expect in the diseased 

 eye a spotted, perforated, or obscured image of the outer world, 

 whereas in fact it is just as bright and clear and complete as 

 in the sound eye, only the whole image appears a trifle reduced 

 in size (so-called micropsis). No physiologist has hitherto been 

 able to offer an explanation of this fact, which apparently defeats 

 all physical laws ; on the contrary, we are forced to acknowledge 

 that we have no knowledge whatever of the psychological act 

 of seeing even in the human eye. Meeting, therefore, in the 

 compound eye with similar difliculties is no reason why we 

 should, on account of some apparent contradictions, doubt 

 observations which may be daily repeated with similar results. 

 In the case of colour-blind people, whose true perception remains 

 absolutely incomprehensible for people with normal sight, we 

 are accustomed to judge their condition from their actions, 

 and to treat all explanations hitherto offered merely as theories 

 not borne out by practice. The same course must for the 

 time being be followed also with regard to the compound insect- 

 eye, of the physiology of which we know nothing whatever. 

 Here it is the experiment we must solely rely on, an experiment 

 we may make on any day and in any place, and for which the 

 first troublesome house-fly may serve as an instructive subject. 



