Eyes and other Objects. 71 



branch of the optic nerve, and its position is exactly 

 at that distance from the lens at which experiment 

 has shown the proper focus would be found. Thus 

 we see in these eyes arrangements closely correspond- 

 ing to the lens, black lining, and retina of our own ; 

 there is also a resemblance to be found to the iris 

 and pupil, as the pigment closes in behind each lens, 

 leaving only a small aperture. And by all these con- 

 trivances, the rays which have passed through the 

 several lenses " are prevented from mixing with each 

 other ; and no rays, save those which pass in the axis 

 of the pyramids, can reach the fibres of the optic 

 nerve/'* It is therefore believed that in looking at 

 an object, an insect can see clearly only the point 

 exactly opposite to the centre of each lens. "The 

 image on the retina may be compared to a mosaic 

 composed of a great number of small images, each of 

 them representing a portion of the object seen. The 

 entire picture is, of course, more perfect in proportion 

 as the pieces are smaller and more numerous.'^ And 

 the reason why the dragon-fly (as well as other insects) 

 has so great a number of eyes may well be concluded 

 to be this that its eyes are not capable of turning 

 round, as ours are, but being supplied with this pro- 

 digious number, the greater portion of its head being 

 studded over with them, it suffers no inconvenience 

 from having them fixed and immovable, as it is thus 

 enabled to see at once in almost all directions. 



The interesting structure, above described, of the 



* Carpenter's " The Microscope and its Revelations," p. 672. 

 f Agassiz and Gould's " Comparative Physiology," p. 70. 



