ABSENCE OF POWER OF ACCOMMODATION. 169 



depend upon the distance o{ ah c from the lens. As 

 a matter of fact, Leydig * and Leuckart t thought they 

 had discovered, between the cornea and the crystalline 

 cones, certain muscular fibres which might regulate 

 the distance between the two, and thus effect this 

 object. Subsequent observers, however, have failed to 

 detect these fibres. 



Again, it will be seen, from a glance at Fig. 76, 

 that in an eye constituted like ours, on the principle 

 of a camera obscura, the retina must follow a regular 

 curve. If it is brought at all too far forward, or forced 

 the least too far back, the image is at once blurred. 

 Hence, in our own case the frequent need for spectacles, 

 and hence it would seem that a conical retina is a 

 physical impossibility. 



Plateau, indeed, adopts t a suggestion made by 

 Grenacher that the absence of any means of adaptation 

 may be rendered unnecessary by the length of the 

 cones, the rays coming from distant objects acting on 

 the anterior end, those from nearer ones at a greater or 

 less depth. This, I confess, seems to me inadmissible. 

 In the first place, the light must surely act immedi- 

 ately it impinges on the organ of perception ; and, in 

 the second, the cones are, as a general rule, abso- 

 lutely transparent — the light passes unimpeded through 

 them. 



Again, if insects see with their compound eyes as we 

 do with ours, they must, of course, possess a retina. 

 No such structure, however, has been as yet show^n to 



* " Zum feineren Bau der Arthropoden," BluUer^s Arch, fur Anat. 

 unci rjiys., 1855. 



t " Carcinologisches," Wiegmann's Arch., 1858. 



X "Rech. Exp. feur la virion chez les Arthropodes," 1887. 



