Eyes of Mollusca and Arthropods. 693 



visual power, as in the Diptera, tlic immber of cases in whieli it is even 

 probable that the eone would prevent the formation of an image is very 

 small. On the other band, in thc Flies, tbere is uudoubtedly very sharp 

 vision , and therefore , if the rhabdom is the percipient dement , we 

 ought to find the lens and eone hig-hly perfected in order , accordiug- 

 to ExNERS supposition , to bring as much light as possible into the 

 extraordinarily small hole at the bottoni of the calyx; the eone is. 

 however, entirely absent, but the lens remains, and is well developed: 

 but alone it must be a positive disadvantage, since, with it, only a 

 single, central ray of light ean possibly enter the rhabdom; all others, 

 since the focus of the lens usually falls in the middle of the calyx , 

 will fall at a sharp angle on the lateral walls of the calyx, never 

 entering the rhabdom. If the calyx and lens wer e entirely ab- 

 sent. or were reduced to a thin layer, — a change that could be accom- 

 plished with the greatest ease , we may be sure , provided there was 

 an advantage to be gained by it , — many more rays of light could 

 gain access to the rhabdom. But as a matter of fact, only a single ray 

 can, under existing circumstances, enter the rhabdom. Now let us 

 eonsider the extraordinarily small number of rays , arising from an 

 ordinary sized object, that could enter a corresponding number of 

 rhabdoms, and the faint impressions that these single rays would pro- 

 duce , and we cannot fail to wonder how these imperfect means could, 

 in so many cases, cause such perfect vision. On the other band I pre- 

 sume the advocates of Müller's theory of mosaic vision would discover, 

 in the presence of the axial nerve fibres which I have shown to be so 

 universally present in the style of the retinophora, a striking confir- 

 mation of their views. The presence of a single nerve dement in the 

 rods was regarded by the followers of Müller, as the fact necessary 

 for the complete confirmation of this theory. Grenacher emphasized 

 this point, but finding in many cases six or seven rods , or nerve Cle- 

 ments, came to the conclusion that, since they were not large enough to 

 receive any entire image , they must act as a single dement, like the 

 rod of a Vertebrate for instance ; if he accepts the fact of an axial nerve 

 fibre, a though bis rhabdoms disappear, bis theory may take a stronger 

 hold than before. 



But those, who, on these grounds, would still retain Müller's 

 theory , will have difficulties to contend with, besides those presented 

 by the crystalline eone. Why for instance is it , if only a single ray of 

 light penetrates the rhabdom, and consequently only extends over 

 the axial nerve, that the rhabdom is, in the presumably most sensi- 



