THE THEORY OF ARTHROPOD VISION. 559 



It is evident if we regard the great rod as a receptive end 

 organ that the light must either act at some part or through- 

 out the whole length of the rod. 



In the latter case the same objections hold good as in Miiller's 

 original theory, such a condition would only give anything 

 like vision with pencils of parallel rays ; the same thing is 

 true if the inner portions of the great rod are the seat of 

 stimulation. 



If the outer extremity of the great rod is the receptive 

 structure, it would receive its illumination from the subcorneal 

 image, and the light rays from every part of this image would 

 fall upon every part of the extremity of the great rod, hence 

 the smallest distinct visual point would be the representative 

 of the subcorneal image. And as this image is similar in all the 

 adjacent facets, there could be no distinct mosaic representing 

 its several parts, and hence all acuity of vision would vanish. 

 Miiller's theory, even in its modified form, is therefore optically 

 untenable. 



Exner's Views. S. Exner has done great service in his optical 

 investigation of the compound eye. He does not ignore, as 

 Notthaft and Grenacher do, the refractive structures, and he 

 has shown that an image may be formed by refraction in the 

 absence of convex surfaces. 



Refractive Cylinders. S. Exner investigated the subcorneal 

 image which exists in the cone of Dytiscus. The corneal 

 facets of this insect are very slightly convex, and he states [252] 

 that the subcorneal image is but little affected by the medium 

 in which the cornea is immersed, a condition which he explains 

 by his theory of refractive cylinders. 



Exner found that the entire crystalline cone of Dytiscus 

 produces an image at a focal distance which led him to deter- 

 mine its refractive index as rS [245]. I found formerly the 

 refractive index for the corneal facet of a Hornet (Vespa crabro) 

 to be equal to 2*0 in a similar manner. Such high refractive 

 indices are unknown amongst organic substances, and indicate 

 an error. Exner [235J, therefore, investigated thin slices of 

 the cone, and found that they have a refractive index equal 



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