092 William Patten 



wliere the cone should be wlien it is absent, exactly 

 similar to those nerve endings in the rods, or percipient 

 elements, of ali other animals; (3) in those animals best 

 able to see, the corneal facets are so constnicted as to 

 throw a perfect image uponthe e ry stalline cone, or upon 

 the centre of the calyx. 



Hence, morphologieally, the seat of vision ought to be in 

 the crystalline cones, the necessary nerves are only to be 

 found in the crystalline cones, and finally the most perfect 

 optical conditions are obtained in the crystalline cones; 

 thereforethe crystalline cones are the percipient elements! 



Accepting this conclusiou , there can be but oue supposition con- 

 cerning the result. In those eyes with lenticiilate facets, an inverted 

 iniage of those objects lying within the axis of the ommatidia will be 

 forme d upon the crystalline cone. In such forms as Mus e a and 

 Mantis, besides mauy others, there is absolutely nothing to 

 prevent the formation of a perfect image, not upon one or 

 two nerve fibres whose surface is in no wise proportional to 

 the size of the image , but upon a complete and perfect series 

 of fibrillae, whose extension in all three directions is suffi- 

 cienttoreceive the wholeofany image forme dby the corneal 

 1 ens. 



The lack of focal accommodation in the lensis balanced 

 by the depth of the retinidium. 



We bave already called attention to the fact that the corneal facets 

 are the products of two or four hypodermic cells. The division between 

 the product of each celi is far less distinct (in ali but exceptional cases, 

 reduced to almost nothing) , than the division between the corresponding 

 facets. These facts indicate that thelatterdivisions,bythegreat 

 difference between their refractive index and that of the 

 surrouuding cuticula, serve to exclude more perfectly 

 lateral rays of light from each crystalline cone. Even if we 

 accept the conclusious of Exner (61), Grenacher, and others, that the 

 crystalline cone would prevent the formation of an image, thatvery 

 conclusion necessit at es the supposition that in the crys- 

 talline cone there would be the greatest accumulation of 

 rays of light! In by far the majority of cases, the shape, consistency 

 and Position of the cone is such as to offer no liindrance to the formation 

 of an image. If we add to such cases those in which no crystalline 

 cone is present and where there is undoubtedly a highly developed 



