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BARKER 



complicated than that of the human eye, in which there are 

 no less than one million cones in a square one-tenth inch. 



The partly diagrammatic sketch of a single ommatidium 

 given below will probably be of use to the reader in locating 

 points touched upon by various authors cited in this paper. 



d. e 



A. Cornea. 



B. Crystalline cone (or pyramid) . 



C. Rliabdoni, represented by a black line, but actually colorless 



and transparent. 



D. Ganglia and optic nerve fibre. 



b. 



d. 



Region of Patten's plexus of nerve fibres. 



Plane in cone where images may sometimes be seen. 



Supposed plane of Miiller's mosaic image aiul Exner's appo- 

 sition image. 



Focal plane of corneal lenses. 



Sensory cells surrounding the rhabdom. These cells are pene- 

 trated by optic nerve fibres according to several writers and 

 are termed the " end organs of vision " by Folsom. 



Retinal plane of some authors. Plane of the superposition 

 image of Exner. 



Cross-section of the rhabdom with enveloping cells, the rhab- 

 dom represented by a black disc to distinguish it from the 

 sheath cells. 



Attention should be called to certain anatomical details 

 not previously mentioned. 



It is commonly stated that the facets of the eye of the 



