302 PTILOTA : INTERNAL ANATOMY. 



bulb or nervous expansion, the impression received by each 

 filament is united to those of the others in the bulb of the 

 optic nerve, and so a common and continuous image is pro- 

 duced. This author, however, further considers, that rays 

 coming from one point of a remote object will illuminate 

 throughout more than a single cone, and thus to each lumi- 

 nous point without there will correspond in the interior of 

 the eye, not exactly a single illuminated point, but rather a 

 little circle of diffused or dispersed light; and in conse- 

 quence an image of but little distinctness will be reproduced 

 on the internal surface or retina, the distinctness of the 

 image increasing in proportion as the object approaches the 

 eye. It is, however, admitted, that the distinctness of the 

 image will increase in proportion to the number of facets 

 and the length of the cones; for the longer the cones are, the 

 more completely will all rays entering them obliquely be pre- 

 vented from reaching their internal extremity or apex. Upon 

 these remarks I would, however, be permitted to observe^ 

 that, from the fact of each facet being the segment of a circle, 

 as well as from the exceedingly minute size of the pupil-like 

 aperture of the pigmental division of each facet, those rays 

 only which fall upon the centre of each facet, and pass down 

 the axis of each cylinder, can reach the nervous filaments in 

 the centre of the eye, and thus there may not be the dis- 

 persion of the rays and consequent indistinctness of vision 

 above noticed. I submit this opinion with great deference, 

 and^^because it appears to me, that in the various experi- 

 ments fmade upon this subject, due allowance has not 

 been made for the external circularity of each facet of the 

 cornea. 



Immediately in connexion with the sense of sight ought 

 to be noticed that remarkable property which many insects 

 possess of emitting a peculiar light, and which, as is appre- 

 hended, must be regarded as displayed by insects with a view 



