574 THE SENSES AND SENSORY ORGANS. 



the cone as parallel pencils p' p' and in in'. If, however, the 

 lenticular surface of the cone is considered, these pencils will 

 unite in focal points at p' in', Fig. 2, D, and the only effect 

 produced by a second refracting surface r r, bounding a denser 

 medium beyond it, will be to increase the convergence and 

 shorten the focus, i.e., reduce the magnifying power of the 

 system. 



Exner gives an elegant proof of the second inversion of the 

 image by the cone [252], to which I must refer the reader who 

 is curious in such matters. 



The Evolution of the Compound Eye. For the above reasons 

 it is clear that the lenticular function of the rhabdome is not 

 necessary except when the crystalline cone is absent, or its 

 focal length is greater than the distance of the retina behind 

 it. In Lampyris the rhabdomes are not apparently lenticular ; 

 in the noctuid Moths there is apparently a large lenticular 

 rhabdome, and in all insects in which the cone is absent the 

 lenticular rhabdome serves as a second refractive medium. 

 \Yc thus see the stages by which a euconic eye, the more 

 primitive form of the compound eye, may have gradually 

 become an aconic eye ; the Moths form an intermediate link, in 

 which both forms of refractive media are present. 



It is possible that, in the most primitive form of compound 

 eye, that of the Phronimidae, total reflection is largely con- 

 cerned in the visual function ; from the thread-like refractive 

 structures of these animals it is not difficult to conceive the 

 origin of refractive cones, such as those of the King-crabs and 

 Crustacea generally. In some insects this kind of eye is but 

 little altered ; in the Moths two lenses, a conical lens and a 

 i h.iUlial lens, appear, with a segregate retina, whilst in the 

 Diptera and Neuroptera the cone has disappeared, the retina 

 has become continuous, and the only refractive media are lenti- 

 cular ; and this may be regarded as the most perfect form of 

 compound eye. 



Such an origin of the various forms of compound eye from 

 a simple type other than that which persists in the ocelli 

 appears probable. The aggregation of simple eyes to form a 



