THE THEORY OF ARTHROPOD VISION. 557 



render the action of the light less effective upon the nerve end 

 organs ? A minute transparent rod with a plane surface and 

 low refractive index would be a much more effective mechanical 

 contrivance. 



Although Grenacher has attempted no further optical 

 analysis of the function of the compound eye, this has been 

 done by Notthaft, who is a great supporter of Grenacher's 

 views. 



Notthaft's Hypothesis. Notthaft [226] shows at considerable 

 length that objects having a surface equal to a square centi- 

 metre are the smallest which produce a distinct visual im- 

 pression at from half a metre to a metre from an insect eye, 

 and he concludes that for all practical purposes, half a metre 

 to a metre is the greatest distance that vision is possible, 

 and even at this distance it is very defective. 



After showing to his own satisfaction that insects are 

 practically blind, he says that it is evident that they are able to 

 guide themselves in their flight, and he explains the manner in 

 which this is accomplished by a very remarkable hypothesis : 

 he supposes that it is only a very thin pencil of parallel rays 

 which excites the retinal end organs ; he arrives at the conclu- 

 sion that the intensity of the illumination of objects, as seen by 

 the insect, decreases with the square of their distance, hence 

 he concludes that it is only near objects which are seen in a 

 dark field of vision, and that insects, when flying, always seek 

 darkness, as in darkness there are no solid objects with which 

 they can come into collision. He seems to forget that black 

 objects would not be seen. 



He admits that insects fly to the light of lamps, and thinks 

 that they do so with the idea of settling on a near solid body 

 when they are tired of seeking darkness. 



These extraordinary views of Notthaft's appear to be a kind 

 of reductio ad absurdum. If Notthaft's theory were accepted, it 

 would follow that darkness and distance are associated sensa- 

 tions, and all dark places or objects would be infinitely distant. 

 Can one imagine a bee seeking the dark entrance of its hive 

 as an infinitely distant object? certainly the idea suggests a 



