26 HUMBLE CREATURES. 



hole, on the same principle as the diaphragm in the 

 eye-piece of a microscope or in the Coddington lens. 



This natural diaphragm is so formed, that the 

 amount of light which is permitted to pass, is to some 

 extent limited, and any remaining tendency to aber- 

 ration in this wonderful instrument is thereby com- 

 pletely corrected. The same layer of dark colouring- 

 matter is continued downwards (PI. IV. fig. 1, d! d' d 

 & fig. 3) between the conical lenses, so that these are 

 effectually isolated, and the rays cannot become con- 

 fused by passing from one lens to the other. The 

 conical lens [b] is curiously shaped, but simple in its 

 structure, not being compound, as is the corneal lens, 

 but of the same density throughout. It is also double 

 convex, the base as well as the apex (from which the 

 point is removed) presenting rounded surfaces. 



At the apex it comes into contact with the bulbous 

 expansion of the optic nerve (PL IV. fig. 1, e), which 

 receives the image of the external object, and this 

 nerve proceeds downward in a line continuous with 

 the axis of the ocellus, until it meets the nerves of 

 the other eyelets (PI. IV. fig. 3). These then unite 

 and form a common trunk that communicates with 

 what we may popularly call the insect's brain (strictly 

 speaking, the " cephalic ganglia," PI. VII. fig. 3, a) . 



But you may, perhaps, be puzzled to understand 

 how so many small images, as must necessarily enter 

 the compound eye of the Bee, can become amalga- 

 mated and combine to form a single picture of the 

 external field ; the effect will, however, be perfectly 



