32$ 



Dr. G. J. Stoney on the Limits of Vision : 



primary surface to tlie inner surface. To make it possible to 

 do this, the glass hemisphere which we have used to represent 

 the primary surface may now be removed ; it is no longer 

 required, since its position is sufficiently indicated by the 

 points of junction of the indiarubber funnels and the glass 



Fig. 3. 



— Outer surface. 



— Primary surface. 



— Inner surface. 



; -- (?) Openings for accommodation 

 muscles. 



--- Nuclear layer. 



— Layer of rods. 



.„ Fibrils of optic nerve. 



Section of an Insect's Eye (diagrammatic). 



threads. The outer surface of our model, which carries the 

 lenses, should be a stiff immovable arch, but the inner sur- 

 face is to be made of some material which is capable of 

 slightly contracting. If, after constructing the model in this 

 way, its inner surface is made to shrink a little*, this will 

 pull the glass threads inwards and elongate all the india- 

 rubber cones. In this way the narrow ends of the cones are 

 brought farther from their lenses, into the position where the 

 image of a near object would be formed. The model now 

 represents the insect's eye when accommodated for the vision 

 of near objects, 



In this model the image of the outer world is formed 

 either at the primary surface or at the inner surface, for a 

 speck of light falling on the upper end of one of the glass 

 threads will travel lengthwise along the thread and emerge 

 from its lower end, being kept from escaping laterally by 



* A diminution of the radius of the inner surface of the model to the 

 extent of about one millimetre would effect a sufficient range of accom- 

 modation. The motion in the insect's eye may need to be more than in 

 proportion to this, since the filaments, as well as the funnels of its eye, 

 are probably extensible, which is not the case in the model, 



