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 veductio 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 
