ON THE EYES OF ARTHROPODS. 197 



view. The great depth at which the percipient structures 

 lie, especially in the euconic eye, added to the fact that the 

 greater part of the picture formed by the lens is shut off 

 from the I'etinula by a dense layer of pigment, which is only 

 performed b}^ a minute opening in the axis of the ocellulus, 

 are diametrically opposed to the dioptric theory : moreover, 

 the depth at which the small image is formed by the corneal 

 facet does not correspond to that of the recipient organ. 

 The further facts in favour of Miiller's theory, adduced by 

 the author, are that the highest perfection of the compound 

 eye is attained in the great multiplication of the number of 

 component ocellulae, and in the fusion of the percipient 

 structures of each into a single organ connected with a 

 single nerve filament ; and that the corneal facets in the 

 Hyperidece are incapable of forming any picture, although 

 no one can suppose the sense of vision in these crustaceans 

 is of an imperfect character. 



Note by the Translator. — Some years ago, whilst in- 

 vestigating the structure of the compound eye of the blow- 

 fly, I was led to take the view that the dioptric theory was 

 the true one; but I am completely convinced, by the valuable 

 researches of Dr. Grenacher, that 1 was wrong in so doing; 

 I never, however, expressed myself very strongly on the 

 point, but distinctly stated that the subject was one which, 

 for a time at least, must remain uncertain. There is one 

 strong point which has not been mentioned by Dr. Grenacher, 

 or as far as 1 know by any one, although it cannot fail to 

 have occurred to the author of this monograph ; it is, that 

 the existence of a large number — many thousand — com- 

 ponents of the picture, each one of which is reversed, would 

 require some very special modification of the nervous 

 apparatus to produce a general picture, the parts of which 

 retain the same relations they possess in the reality. No 

 doubt a decussation of nerve fibres is possible, which should 

 reverse the reversed components of the picture ; each facet, 

 however, of the compound eye has but one nerve fibre in 

 relation with it. On the other hand, in the Diptera at least 

 there is a general decussation of nerve fibres in the great 

 optic nerve of the compound eye ; those from the upper 

 facets crossing those from the lower, and those from the 



