228 



Discoveries of Milller and others in iJie 



with the surrounding pigment. In order to examine the form 

 of the organs under consideration, the eyes should be im- 

 mersed for some time in concentrated alcohol, which hardens 

 without affecting, in general, the transparency of the cones, 

 and so renders them easily distinguishable from the opaque 

 filaments of the optic nerve. 



The form of these crystalline or vitreous cones varies con- 

 siderably in the different articulated animals provided with 

 them. It depends, to a certain extent, but not absolutely, on 

 the form of the facets. In the Crustacea with regular qua- 

 drangular facets, the cones are also quadrangular : in the 

 insects the facets are always hexagonal, but in the Phalae'nae 

 the cones are rounded. Their base, which is in contact, or 

 nearly so, with the corresponding facet of the cornea, is 

 sometimes flat, as in the oyster, sometimes concave ; and 

 generally accords with the internal surface of the facet. The 

 internal extremity of the cone, which is attached to the ex- 

 tremity of the corresponding filament of the optic nerve, is 

 generally pointed ; the point being formed in some abruptly, in 

 others gradually. Their lateral surfaces are separated merely 

 by the interposed pigment which invests each individually. 



The dimensions of the cones also vary very much even in 

 the same eyes. In insects whose eyes deviate from the 

 spherical form, the cones are commonly longer in proportion 

 as the surface of the eye is more flattened, and shorter as the 

 eye is more convex or parabolic. In the different species of 

 the genus Sphinx the diameter of each cone, at its base, is to 

 its longitudinal axis as 1 to 5. The length, in most insects, 

 is very small in proportion to the diameter. In the common 

 oyster it is as 1 to 10. 



If we keep the eyes of a /Sphinx for some time in alcohol, 

 the cones, retaining their transparency, become at length so 

 hard, that they may be detached from each other, and the 

 greater part or the whole of their pigment may be removed 

 by agitating them in water. In taking 

 off the cornea of an eye so hardened, 

 ranges or groups of cones will often be 

 separated from their nervous filaments, 

 and remain adherent to the cornea ; for 

 the texture of these bodies in such cases 

 rarely gives way. Sometimes the fila- 

 ments of the nerves are torn, portions 

 remaining attached to the points of the 

 cones. 



Fig, 44. represents cones insulated and 

 in groups, taken from the eye of aA^phinx, 

 and magnified, a exhibits a group of 



